This experiment devised by Millikan in 1909 used electric fields to deduce the charge on an electron. Fine droplets of oil were sprayed through a hole in a horizontal plate and allowed to drift down through the air between it and a lower plate. As the drops left the spray they became negatively charged and when a potential difference was placed across the plates it was possible to adjust the size of this potential so as to just balance the gravitational pull on the drop.
Suppose a given drop of mass m acquires a charge q1. Then if the potential is adjusted so that the electrostatic force upwards balances the gravitational force downwards then:
where E is the field
between the plates (= V/d where V is the potential difference between them and d their
separation.) Millikan repeated the experiment many times and obtained a series of results for
different droplets with different masses and charges.
He found that all the charges were
multiples of one basic charge namely 1.6x10-19 C and he reasoned that since he found no charge
of smaller value this must be the charge on the electron. It sounds quite an easy experiment but in
fact it is horrendously difficult to perform. Convection currents, varying output voltages from the
power supplies and the difficulty of focusing a microscope on a shivering droplet all make for
problems.
