The charge on the electron - Millikan's oil drop experiment
Oil bath
Oil spray
X ray source
microscope
Charged plates
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: Electrostaticl
attraction
F = Eq
mg = Eq1= q1V/d Oil droplet
Gravitational
attraction
where E is the field between the plates (= V/d where V is the F = mg
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.
Example problem
Calculate the charge on a 27.4 mg oil droplet suspended between two plates 3 mm apart
if the p.d between the plates is 250 V
-6 -3 -9
Using :mg = Eq = qV/d we have q = mgd/V = 27.4x10 x9.8x3x10 /250 = 3.2x10 C