In a series of experiments carried out
between 1908 and 1917, RA Millikan
succeeded in measuring the charge of the
electron with great precision.
In his experiment a fine mist of oil was
sprayed into the upper chamber with an
atomizer. Some of the tiny oil droplets
fell through the hole in the upper floor,
and Millikan was able to determine the
mass of an oil drop from its terminal
velocity.
Next Millikan used an x-ray source to
ionize gas molecules in the chamber.
Electrons from this ionization process
adhered to the oil droplets. The oil
droplets now carry a negative charge.
The negatively charged oil droplets can
be halted by adjusting the voltage
across the two plates. As the voltage
across the plates is increased, the
velocity of the oil drops slows. As the
voltage is increased further some drops
will begin to move upward toward the
positive plate. If the voltage is set
just right an oil drop can be suspended.
When an oil drop is suspended its weight,
mass times acceleration due to gravity,
is exactly counter balanced by the
electric force applied. The electric
force applied equals the applied
electric field (E) times the charge on the
drop (q). Since the mass of the oil drop,
the acceleration due to gravity, and the
applied electric field are known,
Millikan could solve for q which is the
charge on the drop. Millikan found that
droplets had different charges but each
was a whole number multiple of a smaller
charge equal to negative one point six
zero times 10 to the negative 19
coulombs. Millikan concluded that this
was the fundamental unit of charge, the
charge of an electron. From the charge of
an electron and the charge to mass ratio
of an electron determined by Thompson
using a cathode ray tube, Millikan was
able to calculate the mass of an
electron.
The mass of an electron, 9.10 times 10 to
the negative 28 grams, is an exceedingly
small mass
