A Faraday cage or Faraday shield is an enclosure
formed by conducting material or by a mesh
of such material.
Such an enclosure blocks external static and
non-static electric fields by channeling electricity
through the mesh, providing constant voltage
on all sides of the enclosure.
Since the difference in voltage is the measure
of electrical potential, no current flows
through the space.
Faraday cages are named after the English
scientist Michael Faraday, who invented them
in 1836.
A Faraday cage operates because an external
static electrical field causes the electric
charges within the cage's conducting material
to be distributed such that they cancel the
field's effect in the cage's interior.
This phenomenon is used, for example, to protect
electronic equipment from lightning strikes
and electrostatic discharges.
Faraday cages cannot block static or slowly
varying magnetic fields, such as the Earth's
magnetic field.
To a large degree, though, they shield the
interior from external electromagnetic radiation
if the conductor is thick enough and any holes
are significantly smaller than the wavelength
of the radiation.
For example, certain computer forensic test
procedures of electronic systems that require
an environment free of electromagnetic interference
can be carried out within a screen room.
These rooms are spaces that are completely
enclosed by one or more layers of a fine metal
mesh or perforated sheet metal.
The metal layers are grounded to dissipate
any electric currents generated from external
or internal electromagnetic fields, and thus
they block a large amount of the electromagnetic
interference.
See also electromagnetic shielding.
The reception or transmission of radio waves,
a form of electromagnetic radiation, to or
from an antenna within a Faraday cage is heavily
attenuated or blocked by the cage.
History
In 1836, Michael Faraday observed that the
excess charge on a charged conductor resided
only on its exterior and had no influence
on anything enclosed within it.
To demonstrate this fact, he built a room
coated with metal foil and allowed high-voltage
discharges from an electrostatic generator
to strike the outside of the room.
He used an electroscope to show that there
was no electric charge present on the inside
of the room's walls.
Although this cage effect has been attributed
to Michael Faraday's famous ice pail experiments
performed in 1843, it was Benjamin Franklin
in 1755 who observed the effect by lowering
an uncharged cork ball suspended on a silk
thread through an opening in an electrically
charged metal can.
In his words, "the cork was not attracted
to the inside of the can as it would have
been to the outside, and though it touched
the bottom, yet when drawn out it was not
found to be electrified by that touch, as
it would have been by touching the outside.
The fact is singular."
Franklin had discovered the behavior of what
we now refer to as a Faraday cage or shield.
Operation
A Faraday cage is best understood as an approximation
to an ideal hollow conductor.
Externally or internally applied electromagnetic
fields produce forces on the charge carriers
within the conductor; the charges are redistributed
accordingly.
Once the charges have rearranged so as to
cancel the applied field inside, the currents
stop.
If a charge is placed inside an ungrounded
Faraday cage, the internal face of the cage
becomes charged to prevent the existence of
a field inside the body of the cage.
However, this charging of the inner face re-distributes
the charges in the body of the cage.
This charges the outer face of the cage with
a charge equal in sign and magnitude to the
one placed inside the cage.
Since the internal charge and the inner face
cancel each other out, the spread of charges
on the outer face is not affected by the position
of the internal charge inside the cage.
So for all intents and purposes, the cage
generates the same DC electric field that
it would generate if it were simply affected
by the charge placed inside.
The same is not true for electromagnetic waves.
If the cage is grounded, the excess charges
will go to the ground instead of the outer
face, so the inner face and the inner charge
will cancel each other out and the rest of
the cage will retain a neutral charge.
Effectiveness of shielding of a static electric
field depends upon the geometry of the conductive
material.
In the case of a nonlinear varying electric
field, and hence an accompanying varying magnetic
field, the faster the variations are, the
better the material resists penetration, but
on the other hand, the better it passes through
a mesh of given size.
In this case the shielding also depends on
the electrical conductivity of the conductive
materials used in the cages, as well as their
thicknesses.
Examples
A microwave oven utilises a Faraday cage,
which can be partly seen covering the transparent
window, to contain the microwave energy within
the oven.
Elevators and other rooms with metallic conducting
frames famously simulate a Faraday cage effect,
leading to a loss of signal and "dead zones"
for users of cellular phones, radios, and
other electronic devices that require external
electromagnetic signals.
Small, physical Faraday cages are used by
electronics engineers during testing to simulate
such an environment to make sure that the
device gracefully handles these conditions.
The shield of a screened cable, such as USB
cables or the coaxial cable used for cable
television, protects the internal conductors
from external electrical noise and prevents
the RF signals from leaking out.
A booster bag acts as a Faraday cage.
It is often used by shoplifters to steal RFID-tagged
items.
Plastic bags that are impregnated with metal
are used to enclose electronic toll collection
devices during shipment to the customer, so
that a toll charge is not registered if the
delivery truck carrying the item passes through
a toll booth.
Some electrical linemen wear Faraday suits,
which allow them to work on live, high voltage
power lines without risk of electrocution.
The suit prevents electrical current from
flowing through the body, and has no theoretical
voltage limit.
Linemen have successfully worked even the
highest voltage lines safely.
The scan room of a Magnetic Resonance Imaging
machine is designed as a Faraday cage.
This prevents external RF signals from being
added to data collected from the patient,
which would affect the resulting image.
Radiographers are trained to identify the
characteristic artifacts created on images
should the Faraday cage be damaged.
Faraday cages are routinely used in analytical
chemistry to reduce noise while making sensitive
measurements.
A home-made Faraday cage used for simultaneous
microscopy and electrochemistry is shown to
the right.
A Faraday cage was used in 2013 by the Vatican
to shield the Sistine Chapel from electronic
eavesdropping during the secret papal conclave
to elect the next pope.
Automobile and airplane passenger compartments
are essentially Faraday cages, protecting
passengers from electric charges, such as
lightning during a thunderstorm.
See also
Frieder Kempe
Anechoic chamber
Conductive textile
Electromagnetic field
Electromagnetic interference
Mu-metal
References
External links
Faraday Cage Protects from 100,000 V :: Physikshow
Uni Bonn
Notes from physics lecture on Faraday cages
from Michigan State University
Make a Faraday Cage Wallet
Michael Faraday: The Invention of Faraday
Cage background and related experiment
Top Gear's Richard Hammond is protected from
600,000 V by a car.
Top Gear's Richard Hammond as a human lightning
rod - protected by a Voltrex Suit
