Millikan Oil Drop Experiment was designed to obtain the charge of an electron
which would then allow for the calculation of the electronís mass.
J.J. Thomson had been using an almost identical
experiment to that of the Millikan one except he had been using a vapor
cloud for the suspension measurement instead of an oil droplet. The
water vapor made the experiment too difficult because it evaporated quickly.
So Robert A. Millikan of the University
of Chicago used droplets of oil for the suspension measurement.
The way this experiment works is that a
droplet of oil will come out of an atomizer and go through a tiny slit
in an electrode. From the electrode the droplet will pass into a
chamber with an electrode parallel from the electrode it had just passed.
In this chamber Millikan was able to balance or suspend the droplet by
the charge the droplet had picked up when passing through the air.
Usining the amount of voltage needed to suspend the droplet he could then
calculate the charge the the droplet.
on image to view animation
Millikan calculated the force gravity would
have on the droplet and then equaled that to the observed amount of charge
that was required to suspend the droplet to determine the absolute charge
of a particular droplet. When Millikan calculated the force gravity
had on the droplets he very soon realized that the amount of charge on
each minutely affected the velocity of the fall. This was proof of
a very small electron mass. Millikan also noticed that the absolute
charge of each droplet was a multiple of a smallest quanta of charge, 1.6
X 10-19 coulombs. He assumed this to be the charge of
a single electron. Millikan could now calculate the mass of an electron
with Thomsonís ratio of charge to mass (qe/m = 1.76 X 1011 coul/kg)
1.6 X 10-19 coul
1.76 X 1011 coul/kg
m = .091
X 10-30 kg
The electron is apprently has a very small
mass. The mass of the electron is 1836 times smaller than that of
the hydrogen ion. If you multiply 1836 (.091 X 10-30 kg)
you get 1.66 X 10-7 kg which is approximately the value of one
atomic mass unit.