1. Millikan's Oil Drop Experiment charge ? gram 1.76 x 108 coulombs
Mass
1.76 x 108 coulombs
1 gram
? gram
charge
Millikan's
Oil Drop
Experiment

2. Robert Millikan’s contribution to the discovery of atomic structure was preceded by work done by other scientists.
J. J. Thomson, working/playing with a cathode ray tube, discovered properties of the electron.
And although Thomson was not able to figure out the charge of an electron or the mass. He did experimentally derive the relationship between the charge and mass of the electron.

3. By experimenting with the effect of a magnetic field as well as an electric field on the electron J. J. was able to determine the charge to mass ratio.

4. Once the charge to mass ratio had been figured out, then it would be quite easy to figure out the mass of the electron if you knew the charge or vice versa.
Charge
Mass
1.76 x 108 coulombs
1 gram
? gram
charge 6 3 x
Similar to:

5. Millikan’s oil drop experiment
And here’s where Robert Millikan steps in.
The following apparatus
is the famous
Millikan’s oil drop experiment

6. Mist of oil from oil sprayer
Electrically charged plates
Viewing microscope
Source of X-rays
Ionizing radiation

7. The oil drops are ionized as they pass through X-rays
Oil droplets from a perfume mister fall down a small hole in a metal plate
but still fall to the metal plate.

8. The metal plates are charged producing an electric field
The oil drop is suspended in mid-air by the force of repulsion from the negatively charged plate below it.
but still fall to the metal plate.
with the ionizing radiation
Repulsion of like charges

9. From the strength of the electric field generated by the charged plates, Robert Millikan was able calculate the charge on the oil molecules.

10. 3.2 x 10-19 coulombs 4.8 x 10-19 coulombs 6.4 x 10-19 coulombs
Results from the calculations of the charges on the oil drops would look something like this:
3.2 x coulombs
4.8 x coulombs
6.4 x coulombs
8.0 x coulombs
9.6 x coulombs
1.12 x coulombs

11. They are multiples of 1.6 x 10-19.
Some oil drops have more charge than others.
They are multiples of 1.6 x
3.2 x coulombs
4.8 x coulombs
6.4 x coulombs
8.0 x coulombs
9.6 x coulombs
1.12 x coulombs
Calculated charge on oil droplets:

12. Mass of an electron is 9.1 x 10-28 grams
OK, put it all together.
J. J. Thomson figured out the charge to mass ratio of an electron.
Charge
Mass
1.76 x 108 coulombs
1 gram
? gram
charge
1.6 x 10-19
Robert Millikan figured out the charge of an electron.
1.76 x 108 coulombs
1 gram
? gram
1.6x10-19
Mass of an electron is 9.1 x grams