The history of electromagnetic theory begins
with ancient measures to understand atmospheric
electricity, in particular lightning. People
then had little understanding of electricity,
and were unable to explain the phenomena.
Scientific understanding into the nature of
electricity grew throughout the eighteenth
and nineteenth centuries through the work
of researchers such as Ampère, Coulomb, Faraday
and Maxwell.
In the 19th century it had become clear that
electricity and magnetism were related, and
their theories were unified: wherever charges
are in motion electric current results, and
magnetism is due to electric current. The
source for electric field is electric charge,
whereas that for magnetic field is electric
current (charges in motion).
== Ancient and classical history ==
The knowledge of static electricity dates
back to the earliest civilizations, but for
millennia it remained merely an interesting
and mystifying phenomenon, without a theory
to explain its behavior and often confused
with magnetism. The ancients were acquainted
with rather curious properties possessed by
two minerals, amber (Greek: ἤλεκτρον,
ēlektron) and magnetic iron ore (μαγνῆτις
λίθος magnētis lithos, "the Magnesian
stone, lodestone"). Amber, when rubbed, attracts
lightweight objects, such as feathers; magnetic
iron ore has the power of attracting iron.
Based on his find of an Olmec hematite artifact
in Central America, the American astronomer
John Carlson has suggested that "the Olmec
may have discovered and used the geomagnetic
lodestone compass earlier than 1000 BC". If
true, this "predates the Chinese discovery
of the geomagnetic lodestone compass by more
than a millennium". Carlson speculates that
the Olmecs may have used similar artifacts
as a directional device for astrological or
geomantic purposes, or to orient their temples,
the dwellings of the living or the interments
of the dead. The earliest Chinese literature
reference to magnetism lies in a 4th-century
BC book called Book of the Devil Valley Master
(鬼谷子): "The lodestone makes iron come
or it attracts it."
Long before any knowledge of electromagnetism
existed, people were aware of the effects
of electricity. Lightning and other manifestations
of electricity such as St. Elmo's fire were
known in ancient times, but it was not understood
that these phenomena had a common origin.
Ancient Egyptians were aware of shocks when
interacting with electric fish (such as the
electric catfish) or other animals (such as
electric eels). The shocks from animals were
apparent to observers since pre-history by
a variety of peoples that came into contact
with them. Texts from 2750 BC by the ancient
Egyptians referred to these fish as "thunderer
of the Nile" and saw them as the "protectors"
of all the other fish. Another possible approach
to the discovery of the identity of lightning
and electricity from any other source, is
to be attributed to the Arabs, who before
the 15th century used the same Arabic word
for lightning (barq) and the electric ray.Thales
of Miletus, writing at around 600 BC, noted
that rubbing fur on various substances such
as amber would cause them to attract specks
of dust and other light objects. Thales wrote
on the effect now known as static electricity.
The Greeks noted that if they rubbed the amber
for long enough they could even get an electric
spark to jump.
The electrostatic phenomena was again reported
millennia later by Roman and Arabic naturalists
and physicians. Several ancient writers, such
as Pliny the Elder and Scribonius Largus,
attested to the numbing effect of electric
shocks delivered by catfish and torpedo rays.
Pliny in his books writes: "The ancient Tuscans
by their learning hold that there are nine
gods that send forth lightning and those of
eleven sorts." This was in general the early
pagan idea of lightning. The ancients held
some concept that shocks could travel along
conducting objects. Patients suffering from
ailments such as gout or headache were directed
to touch electric fish in the hope that the
powerful jolt might cure them.A number of
objects found in Iraq in 1938 dated to the
early centuries AD (Sassanid Mesopotamia),
called the Baghdad Battery, resembles a galvanic
cell and is believed by some to have been
used for electroplating. The claims are controversial
because of supporting evidence and theories
for the uses of the artifacts, physical evidence
on the objects conducive for electrical functions,
and if they were electrical in nature. As
a result, the nature of these objects is based
on speculation, and the function of these
artifacts remains in doubt.
== Middle Ages and the Renaissance ==
Magnetic attraction was once accounted for
by Aristotle and Thales as the working of
a soul in the stone.
In the 11th century, the Chinese scientist
Shen Kuo (1031–1095) was the first person
to write of the magnetic needle compass and
that it improved the accuracy of navigation
by employing the astronomical concept of true
north (Dream Pool Essays, AD 1088 ), and by
the 12th century the Chinese were known to
use the lodestone compass for navigation.
In 1187, Alexander Neckam was the first in
Europe to describe the compass and its use
for navigation.
In the thirteenth century Peter Peregrinus,
a native of Maricourt in Picardy, made a discovery
of fundamental importance. The French 13th
century scholar conducted experiments on magnetism
and wrote the first extant treatise describing
the properties of magnets and pivoting compass
needles. The dry compass was invented around
1300 by Italian inventor Flavio Gioja.Archbishop
Eustathius of Thessalonica, Greek scholar
and writer of the 12th century, records that
Woliver, king of the Goths, was able to draw
sparks from his body. The same writer states
that a certain philosopher was able while
dressing to draw sparks from his clothes,
a result seemingly akin to that obtained by
Robert Symmer in his silk stocking experiments,
a careful account of which may be found in
the 'Philosophical Transactions,' 1759.Italian
physician Gerolamo Cardano wrote about electricity
in De Subtilitate (1550) distinguishing, perhaps
for the first time, between electrical and
magnetic forces.
== 17th Century ==
Toward the late 16th century, a physician
of Queen Elizabeth's time, Dr. William Gilbert,
in De Magnete, expanded on Cardano's work
and invented the New Latin word electricus
from ἤλεκτρον (ēlektron), the Greek
word for "amber". Gilbert, a native of Colchester,
Fellow of St John's College, Cambridge, and
sometime President of the College of Physicians,
was one of the earliest and most distinguished
English men of science — a man whose work
Galileo thought enviably great. He was appointed
Court physician, and granted a pension to
set him free to continue his research in physics
and chemistry.Gilbert undertook a number of
careful electrical experiments, in the course
of which he discovered that many substances
other than amber, such as sulphur, wax, glass,
etc., were capable of manifesting electrical
properties. Gilbert also discovered that a
heated body lost its electricity and that
moisture prevented the electrification of
all bodies, due to the now well-known fact
that moisture impaired the insulation of such
bodies. He also noticed that electrified substances
attracted all other substances indiscriminately,
whereas a magnet only attracted iron. The
many discoveries of this nature earned for
Gilbert the title of founder of the electrical
science. By investigating the forces on a
light metallic needle, balanced on a point,
he extended the list of electric bodies, and
found also that many substances, including
metals and natural magnets, showed no attractive
forces when rubbed. He noticed that dry weather
with north or east wind was the most favourable
atmospheric condition for exhibiting electric
phenomena—an observation liable to misconception
until the difference between conductor and
insulator was understood.
Gilbert's work was followed up by Robert Boyle
(1627–1691), the famous natural philosopher
who was once described as "father of Chemistry,
and uncle of the Earl of Cork." Boyle was
one of the founders of the Royal Society when
it met privately in Oxford, and became a member
of the Council after the Society was incorporated
by Charles II. in 1663. He worked frequently
at the new science of electricity, and added
several substances to Gilbert's list of electrics.
He left a detailed account of his researches
under the title of Experiments on the Origin
of Electricity. Boyle, in 1675, stated that
electric attraction and repulsion can act
across a vacuum. One of his important discoveries
was that electrified bodies in a vacuum would
attract light substances, thus indicating
that the electrical effect did not depend
upon the air as a medium. He also added resin
to the then known list of electrics.In 1663
Otto von Guericke invented a device that is
now recognized as an early (possibly the first)
electrostatic generator, but he did not recognize
it primarily as an electrical device or conduct
electrical experiments with it. By the end
of the 17th Century, researchers had developed
practical means of generating electricity
by friction with an electrostatic generator,
but the development of electrostatic machines
did not begin in earnest until the 18th century,
when they became fundamental instruments in
the studies about the new science of electricity.
The first usage of the word electricity is
ascribed to Sir Thomas Browne in his 1646
work, Pseudodoxia Epidemica.
The first appearance of the term electromagnetism
on the other hand comes from an earlier date:
1641.
Magnes, by the Jesuit luminary Athanasius
Kircher, carries on page 640 the provocative
chapter-heading: "Elektro-magnetismos i.e.
On the Magnetism of amber, or electrical attractions
and their causes" (ηλεκτρο-μαγνητισμος
id est sive De Magnetismo electri, seu electricis
attractionibus earumque causis).
== 18th century ==
=== 
Improving the electric machine ===
The electric machine was subsequently improved
by Francis Hauksbee, his student Litzendorf,
and by Prof. Georg Matthias Bose, about 1750.
Litzendorf, researching for Christian August
Hausen, substituted a glass ball for the sulphur
ball of Guericke. Bose was the first to employ
the "prime conductor" in such machines, this
consisting of an iron rod held in the hand
of a person whose body was insulated by standing
on a block of resin. Ingenhousz, during 1746,
invented electric machines made of plate glass.
Experiments with the electric machine were
largely aided by the discovery that a glass
plate, coated on both sides with tinfoil,
would accumulate electric charge when connected
with a source of electromotive force. The
electric machine was soon further improved
by Andrew Gordon, a Scotsman, Professor at
Erfurt, who substituted a glass cylinder in
place of a glass globe; and by Giessing of
Leipzig who added a "rubber" consisting of
a cushion of woollen material. The collector,
consisting of a series of metal points, was
added to the machine by Benjamin Wilson about
1746, and in 1762, John Canton of England
(also the inventor of the first pith-ball
electroscope in 1754) improved the efficiency
of electric machines by sprinkling an amalgam
of tin over the surface of the rubber.
=== Electrics and non-electrics ===
In 1729, Stephen Gray conducted a series of
experiments that demonstrated the difference
between conductors and non-conductors (insulators),
showing amongst other things that a metal
wire and even packthread conducted electricity,
whereas silk did not. In one of his experiments
he sent an electric current through 800 feet
of hempen thread which was suspended at intervals
by loops of silk thread. When he tried to
conduct the same experiment substituting the
silk for finely spun brass wire, he found
that the electric current was no longer carried
throughout the hemp cord, but instead seemed
to vanish into the brass wire. From this experiment
he classified substances into two categories:
"electrics" like glass, resin and silk and
"non-electrics" like metal and water. "Non-electrics"
conducted charges while "electrics" held the
charge.
=== Vitreous and resinous ===
Intrigued by Gray's results, in 1732, C. F.
du Fay began to conduct several experiments.
In his first experiment, Du Fay concluded
that all objects except metals, animals, and
liquids could be electrified by rubbing and
that metals, animals and liquids could be
electrified by means of an electric machine,
thus discrediting Gray's "electrics" and "non-electrics"
classification of substances.
In 1737 Du Fay and Hauksbee independently
discovered what they believed to be two kinds
of frictional electricity; one generated from
rubbing glass, the other from rubbing resin.
From this, Du Fay theorized that electricity
consists of two electrical fluids, "vitreous"
and "resinous", that are separated by friction
and that neutralize each other when combined.
This picture of electricity was also supported
by Christian Gottlieb Kratzenstein in his
theoretical and experimental works. The two-fluid
theory would later give rise to the concept
of positive and negative electrical charges
devised by Benjamin Franklin.
=== Leyden jar ===
The Leyden jar, a type of capacitor for electrical
energy in large quantities, was invented independently
by Ewald Georg von Kleist on 11 October 1744
and by Pieter van Musschenbroek in 1745—1746
at Leiden University (the latter location
giving the device its name). William Watson,
when experimenting with the Leyden jar, discovered
in 1747 that a discharge of static electricity
was equivalent to an electric current. Capacitance
was first observed by Von Kleist of Leyden
in 1754. Von Kleist happened to hold, near
his electric machine, a small bottle, in the
neck of which there was an iron nail. Touching
the iron nail accidentally with his other
hand he received a severe electric shock.
In much the same way Musschenbroeck assisted
by Cunaens received a more severe shock from
a somewhat similar glass bottle. Sir William
Watson of England greatly improved this device,
by covering the bottle, or jar, outside and
in with tinfoil. This piece of electrical
apparatus will be easily recognized as the
well-known Leyden jar, so called by the Abbot
Nollet of Paris, after the place of its discovery.In
1741, John Ellicott "proposed to measure the
strength of electrification by its power to
raise a weight in one scale of a balance while
the other was held over the electrified body
and pulled to it by its attractive power".
As early as 1746, Jean-Antoine Nollet (1700–1770)
had performed experiments on the propagation
speed of electricity. By involving 200 monks
connected from hand to hand by a 7-m iron
wire so as to form a circle of about 1.6 km,
he was able to prove that this speed is finite,
even though very high. In 1749, Sir William
Watson conducted numerous experiments to ascertain
the velocity of electricity in a wire. These
experiments, although perhaps not so intended,
also demonstrated the possibility of transmitting
signals to a distance by electricity. In these
experiments, the signal appeared to travel
the 12,276-foot length of the insulated wire
instantaneously. Le Monnier in France had
previously made somewhat similar experiments,
sending shocks through an iron wire 1,319
feet long.About 1750, first experiments in
electrotherapy were made. Various experimenters
made tests to ascertain the physiological
and therapeutical effects of electricity.
Typical for this effort was Kratzenstein in
Halle who in 1744 wrote a treatise on the
subject. Demainbray in Edinburgh examined
the effects of electricity upon plants and
concluded that the growth of two myrtle trees
was quickened by electrification. These myrtles
were electrified "during the whole month of
October, 1746, and they put forth branches
and blossoms sooner than other shrubs of the
same kind not electrified.". Abbé Ménon
in France tried the effects of a continued
application of electricity upon men and birds
and found that the subjects experimented on
lost weight, thus apparently showing that
electricity quickened the excretions. The
efficacy of electric shocks in cases of paralysis
was tested in the county hospital at Shrewsbury,
England, with rather poor success.
=== Late 18th century ===
Benjamin Franklin promoted his investigations
of electricity and theories through the famous,
though extremely dangerous, experiment of
having his son fly a kite through a storm-threatened
sky. A key attached to the kite string sparked
and charged a Leyden jar, thus establishing
the link between lightning and electricity.
Following these experiments, he invented a
lightning rod. It is either Franklin (more
frequently) or Ebenezer Kinnersley of Philadelphia
(less frequently) who is considered to have
established the convention of positive and
negative electricity.
Theories regarding the nature of electricity
were quite vague at this period, and those
prevalent were more or less conflicting. Franklin
considered that electricity was an imponderable
fluid pervading everything, and which, in
its normal condition, was uniformly distributed
in all substances. He assumed that the electrical
manifestations obtained by rubbing glass were
due to the production of an excess of the
electric fluid in that substance and that
the manifestations produced by rubbing wax
were due to a deficit of the fluid. This explanation
was opposed by supporters of the "two-fluid"
theory like Robert Symmer in 1759. In this
theory, the vitreous and resinous electricities
were regarded as imponderable fluids, each
fluid being composed of mutually repellent
particles while the particles of the opposite
electricities are mutually attractive. When
the two fluids unite as a result of their
attraction for one another, their effect upon
external objects is neutralized. The act of
rubbing a body decomposes the fluids, one
of which remains in excess on the body and
manifests itself as vitreous or resinous electricity.Up
to the time of Franklin's historic kite experiment,
the identity of the electricity developed
by rubbing and by electrostatic machines (frictional
electricity) with lightning had not been generally
established. Dr. Wall, Abbot Nollet, Hauksbee,
Stephen Gray and John Henry Winkler had indeed
suggested the resemblance between the phenomena
of "electricity" and "lightning", Gray having
intimated that they only differed in degree.
It was doubtless Franklin, however, who first
proposed tests to determine the sameness of
the phenomena. In a letter to Peter Comlinson
of London, on 19 October 1752, Franklin, referring
to his kite experiment, wrote,
"At this key the phial (Leyden jar) may be
charged; and from the electric fire thus obtained
spirits may be kindled, and all the other
electric experiments be formed which are usually
done by the help of a rubbed glass globe or
tube, and thereby the sameness of the electric
matter with that of lightning be completely
demonstrated."
On 10 May 1742 Thomas-François Dalibard,
at Marley (near Paris), using a vertical iron
rod 40 feet long, obtained results corresponding
to those recorded by Franklin and somewhat
prior to the date of Franklin's experiment.
Franklin's important demonstration of the
sameness of frictional electricity and lightning
doubtless added zest to the efforts of the
many experimenters in this field in the last
half of the 18th century, to advance the progress
of the science.Franklin's observations aided
later scientists such as Michael Faraday,
Luigi Galvani, Alessandro Volta, André-Marie
Ampère and Georg Simon Ohm, whose collective
work provided the basis for modern electrical
technology and for whom fundamental units
of electrical measurement are named. Others
who would advance the field of knowledge included
William Watson, Georg Matthias Bose, Smeaton,
Louis-Guillaume Le Monnier, Jacques de Romas,
Jean Jallabert, Giovanni Battista Beccaria,
Tiberius Cavallo, John Canton, Robert Symmer,
Abbot Nollet, John Henry Winkler, Benjamin
Wilson, Ebenezer Kinnersley, Joseph Priestley,
Franz Aepinus, Edward Hussey Délavai, Henry
Cavendish, and Charles-Augustin de Coulomb.
Descriptions of many of the experiments and
discoveries of these early electrical scientists
may be found in the scientific publications
of the time, notably the Philosophical Transactions,
Philosophical Magazine, Cambridge Mathematical
Journal, Young's Natural Philosophy, Priestley's
History of Electricity, Franklin's Experiments
and Observations on Electricity, Cavalli's
Treatise on Electricity and De la Rive's Treatise
on Electricity.Henry Elles was one of the
first people to suggest links between electricity
and magnetism. In 1757 he claimed that he
had written to the Royal Society in 1755 about
the links between electricity and magnetism,
asserting that "there are some things in the
power of magnetism very similar to those of
electricity" but he did "not by any means
think them the same". In 1760 he similarly
claimed that in 1750 he had been the first
"to think how the electric fire may be the
cause of thunder". Among the more important
of the electrical research and experiments
during this period were those of Franz Aepinus,
a noted German scholar (1724–1802) and Henry
Cavendish of London, England.Franz Aepinus
is credited as the first to conceive of the
view of the reciprocal relationship of electricity
and magnetism. In his work Tentamen Theoria
Electricitatis et Magnetism, published in
Saint Petersburg in 1759, he gives the following
amplification of Franklin's theory, which
in some of its features is measurably in accord
with present-day views: "The particles of
the electric fluid repel each other, attract
and are attracted by the particles of all
bodies with a force that decreases in proportion
as the distance increases; the electric fluid
exists in the pores of bodies; it moves unobstructedly
through non-electric (conductors), but moves
with difficulty in insulators; the manifestations
of electricity are due to the unequal distribution
of the fluid in a body, or to the approach
of bodies unequally charged with the fluid."
Aepinus formulated a corresponding theory
of magnetism excepting that, in the case of
magnetic phenomena, the fluids only acted
on the particles of iron. He also made numerous
electrical experiments apparently showing
that, in order to manifest electrical effects,
tourmaline must be heated to between 37.5°С
and 100 °C. In fact, tourmaline remains unelectrified
when its temperature is uniform, but manifests
electrical properties when its temperature
is rising or falling. Crystals that manifest
electrical properties in this way are termed
pyroelectric; along with tourmaline, these
include sulphate of quinine and quartz.Henry
Cavendish independently conceived a theory
of electricity nearly akin to that of Aepinus.
In 1784, he was perhaps the first to utilize
an electric spark to produce an explosion
of hydrogen and oxygen in the proper proportions
that would create pure water. Cavendish also
discovered the inductive capacity of dielectrics
(insulators), and, as early as 1778, measured
the specific inductive capacity for beeswax
and other substances by comparison with an
air condenser.
Around 1784 C. A. Coulomb devised the torsion
balance, discovering what is now known as
Coulomb's law: the force exerted between two
small electrified bodies varies inversely
as the square of the distance, not as Aepinus
in his theory of electricity had assumed,
merely inversely as the distance. According
to the theory advanced by Cavendish, "the
particles attract and are attracted inversely
as some less power of the distance than the
cube." A large part of the domain of electricity
became virtually annexed by Coulomb's discovery
of the law of inverse squares.
Through the experiments of William Watson
and others proving that electricity could
be transmitted to a distance, the idea of
making practical use of this phenomenon began,
around 1753, to engross the minds of inquisitive
people. To this end, suggestions as to the
employment of electricity in the transmission
of intelligence were made. The first of the
methods devised for this purpose was probably
that of Georges Lesage in 1774. This method
consisted of 24 wires, insulated from one
another and each having had a pith ball connected
to its distant end. Each wire represented
a letter of the alphabet. To send a message,
a desired wire was charged momentarily with
electricity from an electric machine, whereupon
the pith ball connected to that wire would
fly out. Other methods of telegraphing in
which frictional electricity was employed
were also tried, some of which are described
in the history on the telegraph.The era of
galvanic or voltaic electricity represented
a revolutionary break from the historical
focus on frictional electricity. Alessandro
Volta discovered that chemical reactions could
be used to create positively charged anodes
and negatively charged cathodes. When a conductor
was attached between these, the difference
in the electrical potential (also known as
voltage) drove a current between them through
the conductor. The potential difference between
two points is measured in units of volts in
recognition of Volta's work.The first mention
of voltaic electricity, although not recognized
as such at the time, was probably made by
Johann Georg Sulzer in 1767, who, upon placing
a small disc of zinc under his tongue and
a small disc of copper over it, observed a
peculiar taste when the respective metals
touched at their edges. Sulzer assumed that
when the metals came together they were set
into vibration, acting upon the nerves of
the tongue to produce the effects noticed.
In 1790, Prof. Luigi Alyisio Galvani of Bologna,
while conducting experiments on "animal electricity",
noticed the twitching of a frog's legs in
the presence of an electric machine. He observed
that a frog's muscle, suspended on an iron
balustrade by a copper hook passing through
its dorsal column, underwent lively convulsions
without any extraneous cause, the electric
machine being at this time absent.To account
for this phenomenon, Galvani assumed that
electricity of opposite kinds existed in the
nerves and muscles of the frog, the muscles
and nerves constituting the charged coatings
of a Leyden jar. Galvani published the results
of his discoveries, together with his hypothesis,
which engrossed the attention of the physicists
of that time. The most prominent of these
was Volta, professor of physics at Pavia,
who contended that the results observed by
Galvani were the result of the two metals,
copper and iron, acting as electromotors,
and that the muscles of the frog played the
part of a conductor, completing the circuit.
This precipitated a long discussion between
the adherents of the conflicting views. One
group agreed with Volta that the electric
current was the result of an electromotive
force of contact at the two metals; the other
adopted a modification of Galvani's view and
asserted that the current was the result of
a chemical affinity between the metals and
the acids in the pile. Michael Faraday wrote
in the preface to his Experimental Researches,
relative to the question of whether metallic
contact is productive of a part of the electricity
of the voltaic pile: "I see no reason as yet
to alter the opinion I have given; ... but
the point itself is of such great importance
that I intend at the first opportunity renewing
the inquiry, and, if I can, rendering the
proofs either on the one side or the other,
undeniable to all."Even Faraday himself, however,
did not settle the controversy, and while
the views of the advocates on both sides of
the question have undergone modifications,
as subsequent investigations and discoveries
demanded, up to 1918 diversity of opinion
on these points continued to crop out. Volta
made numerous experiments in support of his
theory and ultimately developed the pile or
battery, which was the precursor of all subsequent
chemical batteries, and possessed the distinguishing
merit of being the first means by which a
prolonged continuous current of electricity
was obtainable. Volta communicated a description
of his pile to the Royal Society of London
and shortly thereafter Nicholson and Cavendish
(1780) produced the decomposition of water
by means of the electric current, using Volta's
pile as the source of electromotive force.
== 19th century ==
=== 
Early 19th century ===
In 1800 Alessandro Volta constructed the first
device to produce a large electric current,
later known as the electric battery. Napoleon,
informed of his works, summoned him in 1801
for a command performance of his experiments.
He received many medals and decorations, including
the Légion d'honneur.
Davy in 1806, employing a voltaic pile of
approximately 250 cells, or couples, decomposed
potash and soda, showing that these substances
were respectively the oxides of potassium
and sodium, metals which previously had been
unknown. These experiments were the beginning
of electrochemistry, the investigation of
which Faraday took up, and concerning which
in 1833 he announced his important law of
electrochemical equivalents, viz.: "The same
quantity of electricity — that is, the same
electric current — decomposes chemically
equivalent quantities of all the bodies which
it traverses; hence the weights of elements
separated in these electrolytes are to each
other as their chemical equivalents." Employing
a battery of 2,000 elements of a voltaic pile
Humphry Davy in 1809 gave the first public
demonstration of the electric arc light, using
for the purpose charcoal enclosed in a vacuum.Somewhat
important to note, it was not until many years
after the discovery of the voltaic pile that
the sameness of animal and frictional electricity
with voltaic electricity was clearly recognized
and demonstrated. Thus as late as January
1833 we find Faraday writing in a paper on
the electricity of the electric ray. "After
an examination of the experiments of Walsh,
Ingenhousz, Henry Cavendish, Sir H. Davy,
and Dr. Davy, no doubt remains on my mind
as to the identity of the electricity of the
torpedo with common (frictional) and voltaic
electricity; and I presume that so little
will remain on the mind of others as to justify
my refraining from entering at length into
the philosophical proof of that identity.
The doubts raised by Sir Humphry Davy have
been removed by his brother, Dr. Davy; the
results of the latter being the reverse of
those of the former. ... The general conclusion
which must, I think, be drawn from this collection
of facts (a table showing the similarity,
of properties of the diversely named electricities)
is, that electricity, whatever may be its
source, is identical in its nature."It is
proper to state, however, that prior to Faraday's
time the similarity of electricity derived
from different sources was more than suspected.
Thus, William Hyde Wollaston, wrote in 1801:
"This similarity in the means by which both
electricity and galvanism (voltaic electricity)
appear to be excited in addition to the resemblance
that has been traced between their effects
shows that they are both essentially the same
and confirm an opinion that has already been
advanced by others, that all the differences
discoverable in the effects of the latter
may be owing to its being less intense, but
produced in much larger quantity." In the
same paper Wollaston describes certain experiments
in which he uses very fine wire in a solution
of sulphate of copper through which he passed
electric currents from an electric machine.
This is interesting in connection with the
later day use of almost similarly arranged
fine wires in electrolytic receivers in wireless,
or radio-telegraphy.
In the first half of the 19th century many
very important additions were made to the
world's knowledge concerning electricity and
magnetism. For example, in 1819 Hans Christian
Ørsted of Copenhagen discovered the deflecting
effect of an electric current traversing a
wire upon- a suspended magnetic needle.This
discovery gave a clue to the subsequently
proved intimate relationship between electricity
and magnetism which was promptly followed
up by Ampère who shortly thereafter (1821)
announced his celebrated theory of electrodynamics,
relating to the force that one current exerts
upon another, by its electro-magnetic effects,
namely
Two parallel portions of a circuit attract
one another if the currents in them are flowing
in the same direction, and repel one another
if the currents flow in the opposite direction.
Two portions of circuits crossing one another
obliquely attract one another if both the
currents flow either towards or from the point
of crossing, and repel one another if one
flows to and the other from that point.
When an element of a circuit exerts a force
on another element of a circuit, that force
always tends to urge the second one in a direction
at right angles to its own direction.Ampere
brought a multitude of phenomena into theory
by his investigations of the mechanical forces
between conductors supporting currents and
magnets.
The German physicist Seebeck discovered in
1821 that when heat is applied to the junction
of two metals that had been soldered together
an electric current is set up. This is termed
thermoelectricity. Seebeck's device consists
of a strip of copper bent at each end and
soldered to a plate of bismuth. A magnetic
needle is placed parallel with the copper
strip. When the heat of a lamp is applied
to the junction of the copper and bismuth
an electric current is set up which deflects
the needle.Around this time, Siméon Denis
Poisson attacked the difficult problem of
induced magnetization, and his results, though
differently expressed, are still the theory,
as a most important first approximation. It
was in the application of mathematics to physics
that his services to science were performed.
Perhaps the most original, and certainly the
most permanent in their influence, were his
memoirs on the theory of electricity and magnetism,
which virtually created a new branch of mathematical
physics.
George Green wrote An Essay on the Application
of Mathematical Analysis to the Theories of
Electricity and Magnetism in 1828. The essay
introduced several important concepts, among
them a theorem similar to the modern Green's
theorem, the idea of potential functions as
currently used in physics, and the concept
of what are now called Green's functions.
George Green was the first person to create
a mathematical theory of electricity and magnetism
and his theory formed the foundation for the
work of other scientists such as James Clerk
Maxwell, William Thomson, and others.
Peltier in 1834 discovered an effect opposite
to thermoelectricity, namely, that when a
current is passed through a couple of dissimilar
metals the temperature is lowered or raised
at the junction of the metals, depending on
the direction of the current. This is termed
the Peltier effect. The variations of temperature
are found to be proportional to the strength
of the current and not to the square of the
strength of the current as in the case of
heat due to the ordinary resistance of a conductor.
This second law is the I2R law, discovered
experimentally in 1841 by the English physicist
Joule. In other words, this important law
is that the heat generated in any part of
an electric circuit is directly proportional
to the product of the resistance R of this
part of the circuit and to the square of the
strength of current I flowing in the circuit.In
1822 Johann Schweigger devised the first galvanometer.
This instrument was subsequently much improved
by Wilhelm Weber (1833). In 1825 William Sturgeon
of Woolwich, England, invented the horseshoe
and straight bar electromagnet, receiving
therefor the silver medal of the Society of
Arts. In 1837 Carl Friedrich Gauss and Weber
(both noted workers of this period) jointly
invented a reflecting galvanometer for telegraph
purposes. This was the forerunner of the Thomson
reflecting and other exceedingly sensitive
galvanometers once used in submarine signaling
and still widely employed in electrical measurements.
Arago in 1824 made the important discovery
that when a copper disc is rotated in its
own plane, and if a magnetic needle be freely
suspended on a pivot over the disc, the needle
will rotate with the disc. If on the other
hand the needle is fixed it will tend to retard
the motion of the disc. This effect was termed
Arago's rotations.
Futile attempts were made by Charles Babbage,
Peter Barlow, John Herschel and others to
explain this phenomenon. The true explanation
was reserved for Faraday, namely, that electric
currents are induced in the copper disc by
the cutting of the magnetic lines of force
of the needle, which currents in turn react
on the needle. Georg Simon Ohm did his work
on resistance in the years 1825 and 1826,
and published his results in 1827 as the book
Die galvanische Kette, mathematisch bearbeitet.
He drew considerable inspiration from Fourier's
work on heat conduction in the theoretical
explanation of his work. For experiments,
he initially used voltaic piles, but later
used a thermocouple as this provided a more
stable voltage source in terms of internal
resistance and constant potential difference.
He used a galvanometer to measure current,
and knew that the voltage between the thermocouple
terminals was proportional to the junction
temperature. He then added test wires of varying
length, diameter, and material to complete
the circuit. He found that his data could
be modeled through a simple equation with
variable composed of the reading from a galvanometer,
the length of the test conductor, thermocouple
junction temperature, and a constant of the
entire setup. From this, Ohm determined his
law of proportionality and published his results.
In 1827, he announced the now famous law that
bears his name, that is:
Electromotive force = Current × Resistance
Ohm brought into order a host of puzzling
facts connecting electromotive force and electric
current in conductors, which all previous
electricians had only succeeded in loosely
binding together qualitatively under some
rather vague statements. Ohm found that the
results could be summed up in such a simple
law and by Ohm's discovery a large part of
the domain of electricity became annexed to
theory.
=== Faraday and Henry ===
The discovery of electromagnetic induction
was made almost simultaneously, although independently,
by Michael Faraday, who was first to make
the discovery in 1831, and Joseph Henry in
1832. Henry's discovery of self-induction
and his work on spiral conductors using a
copper coil were made public in 1835, just
before those of Faraday.In 1831 began the
epoch-making researches of Michael Faraday,
the famous pupil and successor of Humphry
Davy at the head of the Royal Institution,
London, relating to electric and electromagnetic
induction. The remarkable researches of Faraday,
the prince of experimentalists, on electrostatics
and electrodynamics and the induction of currents.
These were rather long in being brought from
the crude experimental state to a compact
system, expressing the real essence. Faraday
was not a competent mathematician, but had
he been one, he would have been greatly assisted
in his researches, have saved himself much
useless speculation, and would have anticipated
much later work. He would, for instance, knowing
Ampere's theory, by his own results have readily
been led to Neumann's theory, and the connected
work of Helmholtz and Thomson. Faraday's studies
and researches extended from 1831 to 1855
and a detailed description of his experiments,
deductions and speculations are to be found
in his compiled papers, entitled Experimental
Researches in Electricity.' Faraday was by
profession a chemist. He was not in the remotest
degree a mathematician in the ordinary sense
— indeed it is a question if in all his
writings there is a single mathematical formula.
The experiment which led Faraday to the discovery
of electromagnetic induction was made as follows:
He constructed what is now and was then termed
an induction coil, the primary and secondary
wires of which were wound on a wooden bobbin,
side by side, and insulated from one another.
In the circuit of the primary wire he placed
a battery of approximately 100 cells. In the
secondary wire he inserted a galvanometer.
On making his first test he observed no results,
the galvanometer remaining quiescent, but
on increasing the length of the wires he noticed
a deflection of the galvanometer in the secondary
wire when the circuit of the primary wire
was made and broken. This was the first observed
instance of the development of electromotive
force by electromagnetic induction.He also
discovered that induced currents are established
in a second closed circuit when the current
strength is varied in the first wire, and
that the direction of the current in the secondary
circuit is opposite to that in the first circuit.
Also that a current is induced in a secondary
circuit when another circuit carrying a current
is moved to and from the first circuit, and
that the approach or withdrawal of a magnet
to or from a closed circuit induces momentary
currents in the latter. In short, within the
space of a few months Faraday discovered by
experiment virtually all the laws and facts
now known concerning electro-magnetic induction
and magneto-electric induction. Upon these
discoveries, with scarcely an exception, depends
the operation of the telephone, the dynamo
machine, and incidental to the dynamo electric
machine practically all the gigantic electrical
industries of the world, including electric
lighting, electric traction, the operation
of electric motors for power purposes, and
electro-plating, electrotyping, etc.In his
investigations of the peculiar manner in which
iron filings arrange themselves on a cardboard
or glass in proximity to the poles of a magnet,
Faraday conceived the idea of magnetic "lines
of force" extending from pole to pole of the
magnet and along which the filings tend to
place themselves. On the discovery being made
that magnetic effects accompany the passage
of an electric current in a wire, it was also
assumed that similar magnetic lines of force
whirled around the wire. For convenience and
to account for induced electricity it was
then assumed that when these lines of force
are "cut" by a wire in passing across them
or when the lines of force in rising and falling
cut the wire, a current of electricity is
developed, or to be more exact, an electromotive
force is developed in the wire that sets up
a current in a closed circuit. Faraday advanced
what has been termed the molecular theory
of electricity which assumes that electricity
is the manifestation of a peculiar condition
of the molecule of the body rubbed or the
ether surrounding the body. Faraday also,
by experiment, discovered paramagnetism and
diamagnetism, namely, that all solids and
liquids are either attracted or repelled by
a magnet. For example, iron, nickel, cobalt,
manganese, chromium, etc., are paramagnetic
(attracted by magnetism), whilst other substances,
such as bismuth, phosphorus, antimony, zinc,
etc., are repelled by magnetism or are diamagnetic.Brugans
of Leyden in 1778 and Le Baillif and Becquerel
in 1827 had previously discovered diamagnetism
in the case of bismuth and antimony. Faraday
also rediscovered specific inductive capacity
in 1837, the results of the experiments by
Cavendish not having been published at that
time. He also predicted the retardation of
signals on long submarine cables due to the
inductive effect of the insulation of the
cable, in other words, the static capacity
of the cable. In 1816 telegraph pioneer Francis
Ronalds had also observed signal retardation
on his buried telegraph lines, attributing
it to induction.The 25 years immediately following
Faraday's discoveries of electromagnetic induction
were fruitful in the promulgation of laws
and facts relating to induced currents and
to magnetism. In 1834 Heinrich Lenz and Moritz
von Jacobi independently demonstrated the
now familiar fact that the currents induced
in a coil are proportional to the number of
turns in the coil. Lenz also announced at
that time his important law that, in all cases
of electromagnetic induction the induced currents
have such a direction that their reaction
tends to stop the motion that produces them,
a law that was perhaps deducible from Faraday's
explanation of Arago's rotations.The induction
coil was first designed by Nicholas Callan
in 1836. In 1845 Joseph Henry, the American
physicist, published an account of his valuable
and interesting experiments with induced currents
of a high order, showing that currents could
be induced from the secondary of an induction
coil to the primary of a second coil, thence
to its secondary wire, and so on to the primary
of a third coil, etc. Heinrich Daniel Ruhmkorff
further developed the induction coil, the
Ruhmkorff coil was patented in 1851, and he
utilized long windings of copper wire to achieve
a spark of approximately 2 inches (50 mm)
in length. In 1857, after examining a greatly
improved version made by an American inventor,
Edward Samuel Ritchie, Ruhmkorff improved
his design (as did other engineers), using
glass insulation and other innovations to
allow the production of sparks more than 300
millimetres (12 in) long.
=== Middle 19th century ===
Up to the middle of the 19th century, indeed
up to about 1870, electrical science was,
it may be said, a sealed book to the majority
of electrical workers. Prior to this time
a number of handbooks had been published on
electricity and magnetism, notably Auguste
de La Rive's exhaustive ' Treatise on Electricity,'
in 1851 (French) and 1853 (English); August
Beer's Einleitung in die Elektrostatik, die
Lehre vom Magnetismus und die Elektrodynamik,
Wiedemann's ' Galvanismus,' and Reiss' 'Reibungsal-elektricitat.'
But these works consisted in the main in details
of experiments with electricity and magnetism,
and but little with the laws and facts of
those phenomena. Henry d'Abria published the
results of some researches into the laws of
induced currents, but owing to their complexity
of the investigation it was not productive
of very notable results. Around the mid-19th
century, Fleeming Jenkin's work on ' Electricity
and Magnetism ' and Clerk Maxwell's ' Treatise
on Electricity and Magnetism ' were published.These
books were departures from the beaten path.
As Jenkin states in the preface to his work
the science of the schools was so dissimilar
from that of the practical electrician that
it was quite impossible to give students sufficient,
or even approximately sufficient, textbooks.
A student he said might have mastered de la
Rive's large and valuable treatise and yet
feel as if in an unknown country and listening
to an unknown tongue in the company of practical
men. As another writer has said, with the
coming of Jenkin's and Maxwell's books all
impediments in the way of electrical students
were removed, "the full meaning of Ohm's law
becomes clear; electromotive force, difference
of potential, resistance, current, capacity,
lines of force, magnetization and chemical
affinity were measurable, and could be reasoned
about, and calculations could be made about
them with as much certainty as calculations
in dynamics".About 1850, Kirchhoff published
his laws relating to branched or divided circuits.
He also showed mathematically that according
to the then prevailing electrodynamic theory,
electricity would be propagated along a perfectly
conducting wire with the velocity of light.
Helmholtz investigated mathematically the
effects of induction upon the strength of
a current and deduced therefrom equations,
which experiment confirmed, showing amongst
other important points the retarding effect
of self-induction under certain conditions
of the circuit.
In 1853, Sir William Thomson (later Lord Kelvin)
predicted as a result
of mathematical calculations the oscillatory
nature of the electric discharge of a condenser
circuit. To Henry, however, belongs the credit
of discerning as a result of his experiments
in 1842 the oscillatory nature of the Leyden
jar discharge. He wrote: The phenomena require
us to admit the existence of a principal discharge
in one direction, and then several reflex
actions backward and forward, each more feeble
than the preceding, until the equilibrium
is obtained. These oscillations were subsequently
observed by B. W. Feddersen (1857) who using
a rotating concave mirror projected an image
of the electric spark upon a sensitive plate,
thereby obtaining a photograph of the spark
which plainly indicated the alternating nature
of the discharge. Sir William Thomson was
also the discoverer of the electric convection
of heat (the "Thomson" effect). He designed
for electrical measurements of precision his
quadrant and absolute electrometers. The reflecting
galvanometer and siphon recorder, as applied
to submarine cable signaling, are also due
to him.About 1876 the American physicist Henry
Augustus Rowland of Baltimore demonstrated
the important fact that a static charge carried
around produces the same magnetic effects
as an electric current. The Importance of
this discovery consists in that it may afford
a plausible theory of magnetism, namely, that
magnetism may be the result of directed motion
of rows of molecules carrying static charges.After
Faraday's discovery that electric currents
could be developed in a wire by causing it
to cut across the lines of force of a magnet,
it was to be expected that attempts would
be made to construct machines to avail of
this fact in the development of voltaic currents.
The first machine of this kind was due to
Hippolyte Pixii, 1832. It consisted of two
bobbins of iron wire, opposite which the poles
of a horseshoe magnet were caused to rotate.
As this produced in the coils of the wire
an alternating current, Pixii arranged a commutating
device (commutator) that converted the alternating
current of the coils or armature into a direct
current in the external circuit. This machine
was followed by improved forms of magneto-electric
machines due to Edward Samuel Ritchie, Joseph
Saxton, Edward M. Clarke 1834, Emil Stohrer
1843, Floris Nollet 1849, Shepperd 1856, Van
Maldern, Werner von Siemens, Henry Wilde and
others.A notable advance in the art of dynamo
construction was made by Samuel Alfred Varley
in 1866 and by Siemens and Charles Wheatstone,
who independently discovered that when a coil
of wire, or armature, of the dynamo machine
is rotated between the poles (or in the "field")
of an electromagnet, a weak current is set
up in the coil due to residual magnetism in
the iron of the electromagnet, and that if
the circuit of the armature be connected with
the circuit of the electromagnet, the weak
current developed in the armature increases
the magnetism in the field. This further increases
the magnetic lines of force in which the armature
rotates, which still further increases the
current in the electromagnet, thereby producing
a corresponding increase in the field magnetism,
and so on, until the maximum electromotive
force which the machine is capable of developing
is reached. By means of this principle the
dynamo machine develops its own magnetic field,
thereby much increasing its efficiency and
economical operation. Not by any means, however,
was the dynamo electric machine perfected
at the time mentioned.In 1860 an important
improvement had been made by Dr. Antonio Pacinotti
of Pisa who devised the first electric machine
with a ring armature. This machine was first
used as an electric motor, but afterward as
a generator of electricity. The discovery
of the principle of the reversibility of the
dynamo electric machine (variously attributed
to Walenn 1860; Pacinotti 1864 ; Fontaine,
Gramme 1873; Deprez 1881, and others) whereby
it may be used as an electric motor or as
a generator of electricity has been termed
one of the greatest discoveries of the 19th
century.
In 1872 the drum armature was devised by Hefner-Alteneck.
This machine in a modified form was subsequently
known as the Siemens dynamo. These machines
were presently followed by the Schuckert,
Gulcher, Fein, Brush, Hochhausen, Edison and
the dynamo machines of numerous other inventors.
In the early days of dynamo machine construction
the machines were mainly arranged as direct
current generators, and perhaps the most important
application of such machines at that time
was in electro-plating, for which purpose
machines of low voltage and large current
strength were employed.Beginning about 1887
alternating current generators came into extensive
operation and the commercial development of
the transformer, by means of which currents
of low voltage and high current strength are
transformed to currents of high voltage and
low current strength, and vice versa, in time
revolutionized the transmission of electric
power to long distances. Likewise the introduction
of the rotary converter (in connection with
the "step-down" transformer) which converts
alternating currents into direct currents
(and vice versa) has effected large economies
in the operation of electric power systems.Before
the introduction of dynamo electric machines,
voltaic, or primary, batteries were extensively
used for electro-plating and in telegraphy.
There are two distinct types of voltaic cells,
namely, the "open" and the "closed", or "constant",
type. The open type in brief is that type
which operated on closed circuit becomes,
after a short time, polarized; that is, gases
are liberated in the cell which settle on
the negative plate and establish a resistance
that reduces the current strength. After a
brief interval of open circuit these gases
are eliminated or absorbed and the cell is
again ready for operation. Closed circuit
cells are those in which the gases in the
cells are absorbed as quickly as liberated
and hence the output of the cell is practically
uniform. The Leclanché and Daniell cells,
respectively, are familiar examples of the
"open" and "closed" type of voltaic cell.
Batteries of the Daniell or "gravity" type
were employed almost generally in the United
States and Canada as the source of electromotive
force in telegraphy before the dynamo machine
became available.In the late 19th century,
the term luminiferous aether, meaning light-bearing
aether, was a conjectured medium for the propagation
of light. The word aether stems via Latin
from the Greek αιθήρ, from a root meaning
to kindle, burn, or shine. It signifies the
substance which was thought in ancient times
to fill the upper regions of space, beyond
the clouds.
=== Maxwell ===
In 1864 James Clerk Maxwell of Edinburgh announced
his electromagnetic theory of light, which
was perhaps the greatest single step in the
world's knowledge of electricity. Maxwell
had studied and commented on the field of
electricity and magnetism as early as 1855/6
when On Faraday's lines of force was read
to the Cambridge Philosophical Society. The
paper presented a simplified model of Faraday's
work, and how the two phenomena were related.
He reduced all of the current knowledge into
a linked set of differential equations with
20 equations in 20 variables. This work was
later published as On Physical Lines of Force
in March 1861. In order to determine the force
which is acting on any part of the machine
we must find its momentum, and then calculate
the rate at which this momentum is being changed.
This rate of change will give us the force.
The method of calculation which it is necessary
to employ was first given by Lagrange, and
afterwards developed, with some modifications,
by Hamilton's equations. It is usually referred
to as Hamilton's principle; when the equations
in the original form are used they are known
as Lagrange's equations. Now Maxwell logically
showed how these methods of calculation could
be applied to the electro-magnetic field.
The energy of a dynamical system is partly
kinetic, partly potential. Maxwell supposes
that the magnetic energy of the field is kinetic
energy, the electric energy potential.Around
1862, while lecturing at King's College, Maxwell
calculated that the speed of propagation of
an electromagnetic field is approximately
that of the speed of light. He considered
this to be more than just a coincidence, and
commented "We can scarcely avoid the conclusion
that light consists in the transverse undulations
of the same medium which is the cause of electric
and magnetic phenomena."Working on the problem
further, Maxwell showed that the equations
predict the existence of waves of oscillating
electric and magnetic fields that travel through
empty space at a speed that could be predicted
from simple electrical experiments; using
the data available at the time, Maxwell obtained
a velocity of 310,740,000 m/s. In his 1864
paper A Dynamical Theory of the Electromagnetic
Field, Maxwell wrote, The agreement of the
results seems to show that light and magnetism
are affections of the same substance, and
that light is an electromagnetic disturbance
propagated through the field according to
electromagnetic laws.As already noted herein
Faraday, and before him, Ampère and others,
had inklings that the luminiferous ether of
space was also the medium for electric action.
It was known by calculation and experiment
that the velocity of electricity was approximately
186,000 miles per second; that is, equal to
the velocity of light, which in itself suggests
the idea of a relationship between -electricity
and "light." A number of the earlier philosophers
or mathematicians, as Maxwell terms them,
of the 19th century, held the view that electromagnetic
phenomena were explainable by action at a
distance. Maxwell, following Faraday, contended
that the seat of the phenomena was in the
medium. The methods of the mathematicians
in arriving at their results were synthetical
while Faraday's methods were analytical. Faraday
in his mind's eye saw lines of force traversing
all space where the mathematicians saw centres
of force attracting at a distance. Faraday
sought the seat of the phenomena in real actions
going on in the medium; they were satisfied
that they had found it in a power of action
at a distance on the electric fluids.Both
of these methods, as Maxwell points out, had
succeeded in explaining the propagation of
light as an electromagnetic phenomenon while
at the same time the fundamental conceptions
of what the quantities concerned are, radically
differed. The mathematicians assumed that
insulators were barriers to electric currents;
that, for instance, in a Leyden jar or electric
condenser the electricity was accumulated
at one plate and that by some occult action
at a distance electricity of an opposite kind
was attracted to the other plate.
Maxwell, looking further than Faraday, reasoned
that if light is an electromagnetic phenomenon
and is transmissible through dielectrics such
as glass, the phenomenon must be in the nature
of electromagnetic currents in the dielectrics.
He therefore contended that in the charging
of a condenser, for instance, the action did
not stop at the insulator, but that some "displacement"
currents are set up in the insulating medium,
which currents continue until the resisting
force of the medium equals that of the charging
force. In a closed conductor circuit, an electric
current is also a displacement of electricity.
The conductor offers a certain resistance,
akin to friction, to the displacement of electricity,
and heat is developed in the conductor, proportional
to the square of the current(as already stated
herein), which current flows as long as the
impelling electric force continues. This resistance
may be likened to that met with by a ship
as it displaces in the water in its progress.
The resistance of the dielectric is of a different
nature and has been compared to the compression
of multitudes of springs, which, under compression,
yield with an increasing back pressure, up
to a point where the total back pressure equals
the initial pressure. When the initial pressure
is withdrawn the energy expended in compressing
the "springs" is returned to the circuit,
concurrently with the return of the springs
to their original condition, this producing
a reaction in the opposite direction. Consequently,
the current due to the displacement of electricity
in a conductor may be continuous, while the
displacement currents in a dielectric are
momentary and, in a circuit or medium which
contains but little resistance compared with
capacity or inductance reaction, the currents
of discharge are of an oscillatory or alternating
nature.Maxwell extended this view of displacement
currents in dielectrics to the ether of free
space. Assuming light to be the manifestation
of alterations of electric currents in the
ether, and vibrating at the rate of light
vibrations, these vibrations by induction
set up corresponding vibrations in adjoining
portions of the ether, and in this way the
undulations corresponding to those of light
are propagated as an electromagnetic effect
in the ether. Maxwell's electromagnetic theory
of light obviously involved the existence
of electric waves in free space, and his followers
set themselves the task of experimentally
demonstrating the truth of the theory. By
1871, he presented the Remarks on the mathematical
classification of physical quantities.
=== End of the 19th century ===
In 1887, the German physicist Heinrich Hertz
in a series of experiments proved the actual
existence of electromagnetic waves, showing
that transverse free space electromagnetic
waves can travel over some distance as predicted
by Maxwell and Faraday. Hertz published his
work in a book titled: Electric waves: being
researches on the propagation of electric
action with finite velocity through space.
The discovery of electromagnetic waves in
space led to the development of radio in the
closing years of the 19th century.
The electron as a unit of charge in electrochemistry
was posited by G. Johnstone Stoney in 1874,
who also coined the term electron in 1894.
Plasma was first identified in a Crookes tube,
and so described by Sir William Crookes in
1879 (he called it "radiant matter"). The
place of electricity in leading up to the
discovery of those beautiful phenomena of
the Crookes Tube (due to Sir William Crookes),
viz., Cathode rays, and later to the discovery
of Roentgen or X-rays, must not be overlooked,
since without electricity as the excitant
of the tube the discovery of the rays might
have been postponed indefinitely. It has been
noted herein that Dr. William Gilbert was
termed the founder of electrical science.
This must, however, be regarded as a comparative
statement.
Oliver Heaviside was a self-taught scholar
who reformulated Maxwell's field equations
in terms of electric and magnetic forces and
energy flux, and independently co-formulated
vector analysis.
During the late 1890s a number of physicists
proposed that electricity, as observed in
studies of electrical conduction in conductors,
electrolytes, and cathode ray tubes, consisted
of discrete units, which were given a variety
of names, but the reality of these units had
not been confirmed in a compelling way. However,
there were also indications that the cathode
rays had wavelike properties.Faraday, Weber,
Helmholtz, Clifford and others had glimpses
of this view; and the experimental works of
Zeeman, Goldstein, Crookes, J. J. Thomson
and others had greatly strengthened this view.
Weber predicted that electrical phenomena
were due to the existence of electrical atoms,
the influence of which on one another depended
on their position and relative accelerations
and velocities. Helmholtz and others also
contended that the existence of electrical
atoms followed from Faraday's laws of electrolysis,
and Johnstone Stoney, to whom is due the term
"electron", showed that each chemical ion
of the decomposed electrolyte carries a definite
and constant quantity of electricity, and
inasmuch as these charged ions are separated
on the electrodes as neutral substances there
must be an instant, however brief, when the
charges must be capable of existing separately
as electrical atoms; while in 1887, Clifford
wrote: "There is great reason to believe that
every material atom carries upon it a small
electric current, if it does not wholly consist
of this current."
In 1896, J. J. Thomson performed experiments
indicating that cathode rays really were particles,
found an accurate value for their charge-to-mass
ratio e/m, and found that e/m was independent
of cathode material. He made good estimates
of both the charge e and the mass m, finding
that cathode ray particles, which he called
"corpuscles", had perhaps one thousandth of
the mass of the least massive ion known (hydrogen).
He further showed that the negatively charged
particles produced by radioactive materials,
by heated materials, and by illuminated materials,
were universal. The nature of the Crookes
tube "cathode ray" matter was identified by
Thomson in 1897.In the late 19th century,
the Michelson–Morley experiment was performed
by Albert A. Michelson and Edward W. Morley
at what is now Case Western Reserve University.
It is generally considered to be the evidence
against the theory of a luminiferous aether.
The experiment has also been referred to as
"the kicking-off point for the theoretical
aspects of the Second Scientific Revolution."
Primarily for this work, Michelson was awarded
the Nobel Prize in 1907. Dayton Miller continued
with experiments, conducting thousands of
measurements and eventually developing the
most accurate interferometer in the world
at that time. Miller and others, such as Morley,
continue observations and experiments dealing
with the concepts. A range of proposed aether-dragging
theories could explain the null result but
these were more complex, and tended to use
arbitrary-looking coefficients and physical
assumptions.By the end of the 19th century
electrical engineers had become a distinct
profession, separate from physicists and inventors.
They created companies that investigated,
developed and perfected the techniques of
electricity transmission, and gained support
from governments all over the world for starting
the first worldwide electrical telecommunication
network, the telegraph network. Pioneers in
this field included Werner von Siemens, founder
of Siemens AG in 1847, and John Pender, founder
of Cable & Wireless.
William Stanley made the first public demonstration
of a transformer that enabled commercial delivery
of alternating current in 1886. Large two-phase
alternating current generators were built
by a British electrician, J. E. H. Gordon,
in 1882. Lord Kelvin and Sebastian Ferranti
also developed early alternators, producing
frequencies between 100 and 300 hertz. After
1891, polyphase alternators were introduced
to supply currents of multiple differing phases.
Later alternators were designed for varying
alternating-current frequencies between sixteen
and about one hundred hertz, for use with
arc lighting, incandescent lighting and electric
motors.The possibility of obtaining the electric
current in large quantities, and economically,
by means of dynamo electric machines gave
impetus to the development of incandescent
and arc lighting. Until these machines had
attained a commercial basis voltaic batteries
were the only available source of current
for electric lighting and power. The cost
of these batteries, however, and the difficulties
of maintaining them in reliable operation
were prohibitory of their use for practical
lighting purposes. The date of the employment
of arc and incandescent lamps may be set at
about 1877.Even in 1880, however, but little
headway had been made toward the general use
of these illuminants; the rapid subsequent
growth of this industry is a matter of general
knowledge. The employment of storage batteries,
which were originally termed secondary batteries
or accumulators, began about 1879. Such batteries
are now utilized on a large scale as auxiliaries
to the dynamo machine in electric power-houses
and substations, in electric automobiles and
in immense numbers in automobile ignition
and starting systems, also in fire alarm telegraphy
and other signal systems.
In 1893, the World's Columbian International
Exposition was held in a building which was
devoted to electrical exhibits. General Electric
Company (backed by Edison and J. P. Morgan)
had proposed to power the electric exhibits
with direct current at the cost of one million
dollars. However, Westinghouse proposed to
illuminate the Columbian Exposition in Chicago
with alternating current for half that price,
and Westinghouse won the bid. It was an historical
moment and the beginning of a revolution,
as George Westinghouse introduced the public
to electrical power by illuminating the Exposition.
=== Second Industrial Revolution ===
The Second Industrial Revolution, also known
as the Technological Revolution, was a phase
of rapid industrialization in the final third
of the 19th century and the beginning of the
20th. Along with the expansion of railroads,
iron and steel production, widespread use
of machinery in manufacturing, greatly increased
use of steam power and petroleum, the period
saw expansion in the use electricity and the
adaption of electromagnetic theory in developing
various technologies.
The 1880s saw the spread of large scale commercial
electric power systems, first used for lighting
and eventually for electro-motive power and
heating. Systems early on used alternating
current and direct current. Large centralized
power generation became possible when it was
recognized that alternating current electric
power lines could use transformers to take
advantage of the fact that each doubling of
the voltage would allow the same size cable
to transmit the same amount of power four
times the distance. Transformer were used
to raise voltage at the point of generation
(a representative number is a generator voltage
in the low kilovolt range) to a much higher
voltage (tens of thousands to several hundred
thousand volts) for primary transmission,
followed to several downward transformations,
for commercial and residential domestic use.
Between 1885 and 1890 poly-phase currents
combined with electromagnetic induction and
practical AC induction motors were developed.The
International Electro-Technical Exhibition
of 1891 featuring the long distance transmission
of high-power, three-phase electric current.
It was held between 16 May and 19 October
on the disused site of the three former "Westbahnhöfe"
(Western Railway Stations) in Frankfurt am
Main. The exhibition featured the first long
distance transmission of high-power, three-phase
electric current, which was generated 175
km away at Lauffen am Neckar. As a result
of this successful field trial, three-phase
current became established for electrical
transmission networks throughout the world.Much
was done in the direction in the improvement
of railroad terminal facilities, and it is
difficult to find one steam railroad engineer
who would have denied that all the important
steam railroads of this country were not to
be operated electrically. In other directions
the progress of events as to the utilization
of electric power was expected to be equally
rapid. In every part of the world the power
of falling water, nature's perpetual motion
machine, which has been going to waste since
the world began, is now being converted into
electricity and transmitted by wire hundreds
of miles to points where it is usefully and
economically employed.The first windmill for
electricity production was built in Scotland
in July 1887 by the Scottish electrical engineer
James Blyth. Across the Atlantic, in Cleveland,
Ohio a larger and heavily engineered machine
was designed and constructed in 1887–88
by Charles F. Brush, this was built by his
engineering company at his home and operated
from 1886 until 1900. The Brush wind turbine
had a rotor 56 feet (17 m) in diameter and
was mounted on a 60-foot (18 m) tower. Although
large by today's standards, the machine was
only rated at 12 kW; it turned relatively
slowly since it had 144 blades. The connected
dynamo was used either to charge a bank of
batteries or to operate up to 100 incandescent
light bulbs, three arc lamps, and various
motors in Brush's laboratory. The machine
fell into disuse after 1900 when electricity
became available from Cleveland's central
stations, and was abandoned in 1908.
== 20th century ==
Various units of electricity and magnetism
have been adopted and named by representatives
of the electrical engineering institutes of
the world, which units and names have been
confirmed and legalized by the governments
of the United States and other countries.
Thus the volt, from the Italian Volta, has
been adopted as the practical unit of electromotive
force, the ohm, from the enunciator of Ohm's
law, as the practical unit of resistance;
the ampere, after the eminent French scientist
of that name, as the practical unit of current
strength, the henry as the practical unit
of inductance, after Joseph Henry and in recognition
of his early and important experimental work
in mutual induction.Dewar and John Ambrose
Fleming predicted that at absolute zero, pure
metals would become perfect electromagnetic
conductors (though, later, Dewar altered his
opinion on the disappearance of resistance
believing that there would always be some
resistance). Walther Hermann Nernst developed
the third law of thermodynamics and stated
that absolute zero was unattainable. Carl
von Linde and William Hampson, both commercial
researchers, nearly at the same time filed
for patents on the Joule–Thomson effect.
Linde's patent was the climax of 20 years
of systematic investigation of established
facts, using a regenerative counterflow method.
Hampson's design was also of a regenerative
method. The combined process became known
as the Linde–Hampson liquefaction process.
Heike Kamerlingh Onnes purchased a Linde machine
for his research. Zygmunt Florenty Wróblewski
conducted research into electrical properties
at low temperatures, though his research ended
early due to his accidental death. Around
1864, Karol Olszewski and Wroblewski predicted
the electrical phenomena of dropping resistance
levels at ultra-cold temperatures. Olszewski
and Wroblewski documented evidence of this
in the 1880s. A milestone was achieved on
10 July 1908 when Onnes at the Leiden University
in Leiden produced, for the first time, liquified
helium and achieved superconductivity.
In 1900, William Du Bois Duddell develops
the Singing Arc and produced melodic sounds,
from a low to a high-tone, from this arc lamp.
=== Lorentz and Poincaré ===
Between 1900 and 1910, many scientists like
Wilhelm Wien, Max Abraham, Hermann Minkowski,
or Gustav Mie believed that all forces of
nature are of electromagnetic origin (the
so-called "electromagnetic world view"). This
was connected with the electron theory developed
between 1892 and 1904 by Hendrik Lorentz.
Lorentz introduced a strict separation between
matter (electrons) and the aether, whereby
in his model the ether is completely motionless,
and it won't be set in motion in the neighborhood
of ponderable matter. Contrary to other electron
models before, the electromagnetic field of
the ether appears as a mediator between the
electrons, and changes in this field can propagate
not faster than the speed of light.
In 1896, three years after submitting his
thesis on the Kerr effect, Pieter Zeeman disobeyed
the direct orders of his supervisor and used
laboratory equipment to measure the splitting
of spectral lines by a strong magnetic field.
Lorentz theoretically explained the Zeeman
effect on the basis of his theory, for which
both received the Nobel Prize in Physics in
1902. A fundamental concept of Lorentz's theory
in 1895 was the "theorem of corresponding
states" for terms of order v/c. This theorem
states that a moving observer (relative to
the ether) makes the same observations as
a resting observer. This theorem was extended
for terms of all orders by Lorentz in 1904.
Lorentz noticed, that it was necessary to
change the space-time variables when changing
frames and introduced concepts like physical
length contraction (1892) to explain the Michelson–Morley
experiment, and the mathematical concept of
local time (1895) to explain the aberration
of light and the Fizeau experiment. That resulted
in the formulation of the so-called Lorentz
transformation by Joseph Larmor (1897, 1900)
and Lorentz (1899, 1904). As Lorentz later
noted (1921, 1928), he considered the time
indicated by clocks resting in the aether
as "true" time, while local time was seen
by him as a heuristic working hypothesis and
a mathematical artifice. Therefore, Lorentz's
theorem is seen by modern historians as being
a mathematical transformation from a "real"
system resting in the aether into a "fictitious"
system in motion.
Continuing the work of Lorentz, Henri Poincaré
between 1895 and 1905 formulated on many occasions
the principle of relativity and tried to harmonize
it with electrodynamics. He declared simultaneity
only a convenient convention which depends
on the speed of light, whereby the constancy
of the speed of light would be a useful postulate
for making the laws of nature as simple as
possible. In 1900 he interpreted Lorentz's
local time as the result of clock synchronization
by light signals, and introduced the electromagnetic
momentum by comparing electromagnetic energy
to what he called a "fictitious fluid" of
mass
m
=
E
/
c
2
{\displaystyle m=E/c^{2}}
. And finally in June and July 1905 he declared
the relativity principle a general law of
nature, including gravitation. He corrected
some mistakes of Lorentz and proved the Lorentz
covariance of the electromagnetic equations.
Poincaré also suggested that there exist
non-electrical forces to stabilize the electron
configuration and asserted that gravitation
is a non-electrical force as well, contrary
to the electromagnetic world view. However,
historians pointed out that he still used
the notion of an ether and distinguished between
"apparent" and "real" time and therefore didn't
invent special relativity in its modern understanding.
=== Einstein's Annus Mirabilis ===
In 1905, while he was working in the patent
office, Albert Einstein had four papers published
in the Annalen der Physik, the leading German
physics journal. These are the papers that
history has come to call the Annus Mirabilis
papers:
His paper on the particulate nature of light
put forward the idea that certain experimental
results, notably the photoelectric effect,
could be simply understood from the postulate
that light interacts with matter as discrete
"packets" (quanta) of energy, an idea that
had been introduced by Max Planck in 1900
as a purely mathematical manipulation, and
which seemed to contradict contemporary wave
theories of light (Einstein 1905a). This was
the only work of Einstein's that he himself
called "revolutionary."
His paper on Brownian motion explained the
random movement of very small objects as direct
evidence of molecular action, thus supporting
the atomic theory. (Einstein 1905b)
His paper on the electrodynamics of moving
bodies introduced the radical theory of special
relativity, which showed that the observed
independence of the speed of light on the
observer's state of motion required fundamental
changes to the notion of simultaneity. Consequences
of this include the time-space frame of a
moving body slowing down and contracting (in
the direction of motion) relative to the frame
of the observer. This paper also argued that
the idea of a luminiferous aether—one of
the leading theoretical entities in physics
at the time—was superfluous. (Einstein 1905c)
In his paper on mass–energy equivalence
(previously considered to be distinct concepts),
Einstein deduced from his equations of special
relativity what later became the well-known
expression:
E
=
m
c
2
{\displaystyle E=mc^{2}}
, suggesting that tiny amounts of mass could
be converted into huge amounts of energy.
(Einstein 1905d)All four papers are today
recognized as tremendous achievements—and
hence 1905 is known as Einstein's "Wonderful
Year". At the time, however, they were not
noticed by most physicists as being important,
and many of those who did notice them rejected
them outright. Some of this work—such as
the theory of light quanta—remained controversial
for years.
=== Mid-20th century ===
The first formulation of a quantum theory
describing radiation and matter interaction
is due to Paul Dirac, who, during 1920, was
first able to compute the coefficient of spontaneous
emission of an atom. Paul Dirac described
the quantization of the electromagnetic field
as an ensemble of harmonic oscillators with
the introduction of the concept of creation
and annihilation operators of particles. In
the following years, with contributions from
Wolfgang Pauli, Eugene Wigner, Pascual Jordan,
Werner Heisenberg and an elegant formulation
of quantum electrodynamics due to Enrico Fermi,
physicists came to believe that, in principle,
it would be possible to perform any computation
for any physical process involving photons
and charged particles. However, further studies
by Felix Bloch with Arnold Nordsieck, and
Victor Weisskopf, in 1937 and 1939, revealed
that such computations were reliable only
at a first order of perturbation theory, a
problem already pointed out by Robert Oppenheimer.
At higher orders in the series infinities
emerged, making such computations meaningless
and casting serious doubts on the internal
consistency of the theory itself. With no
solution for this problem known at the time,
it appeared that a fundamental incompatibility
existed between special relativity and quantum
mechanics.
In December 1938, the German chemists Otto
Hahn and Fritz Strassmann sent a manuscript
to Naturwissenschaften reporting they had
detected the element barium after bombarding
uranium with neutrons; simultaneously, they
communicated these results to Lise Meitner.
Meitner, and her nephew Otto Robert Frisch,
correctly interpreted these results as being
nuclear fission. Frisch confirmed this experimentally
on 13 January 1939. In 1944, Hahn received
the Nobel Prize in Chemistry for the discovery
of nuclear fission. Some historians who have
documented the history of the discovery of
nuclear fission believe Meitner should have
been awarded the Nobel Prize with Hahn.Difficulties
with the Quantum theory increased through
the end of 1940. Improvements in microwave
technology made it possible to take more precise
measurements of the shift of the levels of
a hydrogen atom, now known as the Lamb shift
and magnetic moment of the electron. These
experiments unequivocally exposed discrepancies
which the theory was unable to explain. With
the invention of bubble chambers and spark
chambers in the 1950s, experimental particle
physics discovered a large and ever-growing
number of particles called hadrons. It seemed
that such a large number of particles could
not all be fundamental.
Shortly after the end of the war in 1945,
Bell Labs formed a Solid State Physics Group,
led by William Shockley and chemist Stanley
Morgan; other personnel including John Bardeen
and Walter Brattain, physicist Gerald Pearson,
chemist Robert Gibney, electronics expert
Hilbert Moore and several technicians. Their
assignment was to seek a solid-state alternative
to fragile glass vacuum tube amplifiers. Their
first attempts were based on Shockley's ideas
about using an external electrical field on
a semiconductor to affect its conductivity.
These experiments failed every time in all
sorts of configurations and materials. The
group was at a standstill until Bardeen suggested
a theory that invoked surface states that
prevented the field from penetrating the semiconductor.
The group changed its focus to study these
surface states and they met almost daily to
discuss the work. The rapport of the group
was excellent, and ideas were freely exchanged.As
to the problems in the electron experiments,
a path to a solution was given by Hans Bethe.
In 1947, while he was traveling by train to
reach Schenectady from New York, after giving
a talk at the conference at Shelter Island
on the subject, Bethe completed the first
non-relativistic computation of the shift
of the lines of the hydrogen atom as measured
by Lamb and Retherford. Despite the limitations
of the computation, agreement was excellent.
The idea was simply to attach infinities to
corrections at mass and charge that were actually
fixed to a finite value by experiments. In
this way, the infinities get absorbed in those
constants and yield a finite result in good
agreement with experiments. This procedure
was named renormalization.
Based on Bethe's intuition and fundamental
papers on the subject by Shin'ichirō Tomonaga,
Julian Schwinger, Richard Feynman and Freeman
Dyson, it was finally possible to get fully
covariant formulations that were finite at
any order in a perturbation series of quantum
electrodynamics. Shin'ichirō Tomonaga, Julian
Schwinger and Richard Feynman were jointly
awarded with a Nobel Prize in Physics in 1965
for their work in this area. Their contributions,
and those of Freeman Dyson, were about covariant
and gauge-invariant formulations of quantum
electrodynamics that allow computations of
observables at any order of perturbation theory.
Feynman's mathematical technique, based on
his diagrams, initially seemed very different
from the field-theoretic, operator-based approach
of Schwinger and Tomonaga, but Freeman Dyson
later showed that the two approaches were
equivalent. Renormalization, the need to attach
a physical meaning at certain divergences
appearing in the theory through integrals,
has subsequently become one of the fundamental
aspects of quantum field theory and has come
to be seen as a criterion for a theory's general
acceptability. Even though renormalization
works very well in practice, Feynman was never
entirely comfortable with its mathematical
validity, even referring to renormalization
as a "shell game" and "hocus pocus". QED has
served as the model and template for all subsequent
quantum field theories. Peter Higgs, Jeffrey
Goldstone, and others, Sheldon Glashow, Steven
Weinberg and Abdus Salam independently showed
how the weak nuclear force and quantum electrodynamics
could be merged into a single electroweak
force.
Robert Noyce credited Kurt Lehovec for the
principle of p–n junction isolation caused
by the action of a biased p-n junction (the
diode) as a key concept behind the integrated
circuit. Jack Kilby recorded his initial ideas
concerning the integrated circuit in July
1958 and successfully demonstrated the first
working integrated circuit on September 12,
1958. In his patent application of February
6, 1959, Kilby described his new device as
"a body of semiconductor material ... wherein
all the components of the electronic circuit
are completely integrated." Kilby won the
2000 Nobel Prize in Physics for his part of
the invention of the integrated circuit. Robert
Noyce also came up with his own idea of an
integrated circuit half a year later than
Kilby. Noyce's chip solved many practical
problems that Kilby's had not. Noyce's chip,
made at Fairchild Semiconductor, was made
of silicon, whereas Kilby's chip was made
of germanium.
Philo Farnsworth developed the Farnsworth–Hirsch
Fusor, or simply fusor, an apparatus designed
by Farnsworth to create nuclear fusion. Unlike
most controlled fusion systems, which slowly
heat a magnetically confined plasma, the fusor
injects high temperature ions directly into
a reaction chamber, thereby avoiding a considerable
amount of complexity. When the Farnsworth-Hirsch
Fusor was first introduced to the fusion research
world in the late 1960s, the Fusor was the
first device that could clearly demonstrate
it was producing fusion reactions at all.
Hopes at the time were high that it could
be quickly developed into a practical power
source. However, as with other fusion experiments,
development into a power source has proven
difficult. Nevertheless, the fusor has since
become a practical neutron source and is produced
commercially for this role.
=== Parity violation ===
The mirror image of an electromagnet produces
a field with the opposite polarity. Thus the
north and south poles of a magnet have the
same symmetry as left and right. Prior to
1956, it was believed that this symmetry was
perfect, and that a technician would be unable
to distinguish the north and south poles of
a magnet except by reference to left and right.
In that year, T. D. Lee and C. N. Yang predicted
the nonconservation of parity in the weak
interaction. To the surprise of many physicists,
in 1957 C. S. Wu and collaborators at the
U.S. National Bureau of Standards demonstrated
that under suitable conditions for polarization
of nuclei, the beta decay of cobalt-60 preferentially
releases electrons toward the south pole of
an external magnetic field, and a somewhat
higher number of gamma rays toward the north
pole. As a result, the experimental apparatus
does not behave comparably with its mirror
image.
=== Electroweak theory ===
The first step towards the Standard Model
was Sheldon Glashow's discovery, in 1960,
of a way to combine the electromagnetic and
weak interactions. In 1967, Steven Weinberg
and Abdus Salam incorporated the Higgs mechanism
into Glashow's electroweak theory, giving
it its modern form. The Higgs mechanism is
believed to give rise to the masses of all
the elementary particles in the Standard Model.
This includes the masses of the W and Z bosons,
and the masses of the fermions - i.e. the
quarks and leptons. After the neutral weak
currents caused by Z boson exchange were discovered
at CERN in 1973, the electroweak theory became
widely accepted and Glashow, Salam, and Weinberg
shared the 1979 Nobel Prize in Physics for
discovering it. The W and Z bosons were discovered
experimentally in 1981, and their masses were
found to be as the Standard Model predicted.
The theory of the strong interaction, to which
many contributed, acquired its modern form
around 1973–74, when experiments confirmed
that the hadrons were composed of fractionally
charged quarks. With the establishment of
quantum chromodynamics in the 1970s finalized
a set of fundamental and exchange particles,
which allowed for the establishment of a "standard
model" based on the mathematics of gauge invariance,
which successfully described all forces except
for gravity, and which remains generally accepted
within the domain to which it is designed
to be applied.
The 'standard model' groups the electroweak
interaction theory and quantum chromodynamics
into a structure denoted by the gauge group
SU(3)×SU(2)×U(1). The formulation of the
unification of the electromagnetic and weak
interactions in the standard model is due
to Abdus Salam, Steven Weinberg and, subsequently,
Sheldon Glashow. After the discovery, made
at CERN, of the existence of neutral weak
currents, mediated by the Z boson foreseen
in the standard model, the physicists Salam,
Glashow and Weinberg received the 1979 Nobel
Prize in Physics for their electroweak theory.
Since then, discoveries of the bottom quark
(1977), the top quark (1995) and the tau neutrino
(2000) have given credence to the standard
model. Because of its success in explaining
a wide variety of experimental results.
== 21st century ==
=== 
Electromagnetic technologies ===
There are a range of emerging energy technologies.
By 2007, solid state micrometer-scale electric
double-layer capacitors based on advanced
superionic conductors had been for low-voltage
electronics such as deep-sub-voltage nanoelectronics
and related technologies (the 22 nm technological
node of CMOS and beyond). Also, the nanowire
battery, a lithium-ion battery, was invented
by a team led by Dr. Yi Cui in 2007.
==== Magnetic resonance ====
Reflecting the fundamental importance and
applicability of Magnetic resonance imaging
in medicine, Paul Lauterbur of the University
of Illinois at Urbana-Champaign and Sir Peter
Mansfield of the University of Nottingham
were awarded the 2003 Nobel Prize in Physiology
or Medicine for their "discoveries concerning
magnetic resonance imaging". The Nobel citation
acknowledged Lauterbur's insight of using
magnetic field gradients to determine spatial
localization, a discovery that allowed rapid
acquisition of 2D images.
==== Wireless electricity ====
Wireless electricity is a form of wireless
energy transfer, the ability to provide electrical
energy to remote objects without wires. The
term WiTricity was coined in 2005 by Dave
Gerding and later used for a project led by
Prof. Marin Soljačić in 2007. The MIT researchers
successfully demonstrated the ability to power
a 60 watt light bulb wirelessly, using two
5-turn copper coils of 60 cm (24 in) diameter,
that were 2 m (7 ft) away, at roughly 45%
efficiency. This technology can potentially
be used in a large variety of applications,
including consumer, industrial, medical and
military. Its aim is to reduce the dependence
on batteries. Further applications for this
technology include transmission of information—it
would not interfere with radio waves and thus
could be used as a cheap and efficient communication
device without requiring a license or a government
permit.
=== Unified theories ===
A Grand Unified Theory (GUT) is a model in
particle physics in which, at high energy,
the electromagnetic force is merged with the
other two gauge interactions of the Standard
Model, the weak and strong nuclear forces.
Many candidates have been proposed, but none
is directly supported by experimental evidence.
GUTs are often seen as intermediate steps
towards a "Theory of Everything" (TOE), a
putative theory of theoretical physics that
fully explains and links together all known
physical phenomena, and, ideally, has predictive
power for the outcome of any experiment that
could be carried out in principle. No such
theory has yet been accepted by the physics
community.
=== Open problems ===
The magnetic monopole in the quantum theory
of magnetic charge started with a paper by
the physicist Paul A.M. Dirac in 1931. The
detection of magnetic monopoles is an open
problem in experimental physics. In some theoretical
models, magnetic monopoles are unlikely to
be observed, because they are too massive
to be created in particle accelerators, and
also too rare in the Universe to enter a particle
detector with much probability.
After more than twenty years of intensive
research, the origin of high-temperature superconductivity
is still not clear, but it seems that instead
of electron-phonon attraction mechanisms,
as in conventional superconductivity, one
is dealing with genuine electronic mechanisms
(e.g. by antiferromagnetic correlations),
and instead of s-wave pairing, d-wave pairings
are substantial. One goal of all this research
is room-temperature superconductivity.
== See also ==
Histories
History of electromagnetic spectrum, History
of electrical engineering, History of Maxwell's
equations, History of radio, History of optics,
History of physics
General
Biot–Savart law, Ponderomotive force, Telluric
currents, Terrestrial magnetism, ampere-hours,
Transverse waves, Longitudinal waves, Plane
waves, Refractive index, torque, Revolutions
per minute, Photosphere, Vortex, vortex rings,
Theory
permittivity, scalar product, vector product,
tensor, divergent series, linear operator,
unit vector, parallelepiped, osculating plane,
standard candle
Technology
Solenoid, electro-magnets, Nicol prisms, rheostat,
voltmeter, gutta-percha covered wire, Electrical
conductor, ammeters, Gramme machine, binding
posts, Induction motor, Lightning arresters,
Technological and industrial history of the
United States, Western Electric Company,
Lists
Outline of energy development
Timelines
Timeline of electromagnetism, Timeline of
luminiferous aether
