Millikan oil drop experiment–Apparatus
Just as all the words in the English language are combinations of subsets of 26 letters, atomic physics showed that atoms of the many elements are combinations of three particles–the proton, neutron and electron. The Millikan oil–drop experiment is a famous experiment done in 1909 that measured the magnitude of the charge on the electron. Millikan got the Nobel Prize in 1923 for this work.
An atomizer sprayed a fine mist of oil droplets into the chamber. Some of these tiny droplets fell through a hole in the upper floor. Millikan first let them fall until they reached terminal velocity. Using the microscope, he measured their terminal velocity, and by use of a formula, calculated the mass of each oil drop. Next, Millikan applied a charge to the falling drops by illuminating the bottom chamber with x–rays. This caused the air to become ionized, and electrons to attach themselves to the oil drops.
By attaching a battery to the plates above and below this bottom chamber, he was able to apply an electric voltage. The electric field produced in the bottom chamber by this voltage would act on the charged oil drops; if the voltage was just right, the electromagnetic force would just balance the force of gravity on a drop, and the drop would hang suspended in mid–air.
When a drop is suspended, its weight m.g is exactly equal to the electric force applied q.E.
The values of E, the applied electric field, m the mass of a drop, and g, the acceleration due to gravity, are all known values. So you can solve for q, the charge on the drop.
Millikan determined the charge on a drop. Then he repeated the experiment numerous times, each time varying the strength of the x–rays ionizing the air, so that differing numbers of electrons would jump onto the oil molecules each time. He obtained various values for q.
The charge q on a drop was always a multiple of −1.6 × 10−19 C, the charge on a single electron.
From these data J.J Thomson calculated the mass of an electron: which he found out to be an exceedingly small
value.
