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

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.

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.

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:

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

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

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.

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

The X-rays contain high energy which will knock electrons out of the oil molecule. The oil drops become positively charged. From the strength of the electric field generated by the charged plates, Robert Millikan was able calculate the charge on the oil molecules.

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 10-19 coulombs 4.8 x 10-19 coulombs 6.4 x 10-19 coulombs 8.0 x 10-19 coulombs 9.6 x 10-19 coulombs 1.12 x 10-18 coulombs

They are multiples of 1.6 x 10-19. Remember, the X-rays striking an oil droplet would remove electrons. They are multiples of 1.6 x 10-19. Some oil drops lost more electrons than others. 3.2 x 10-19 coulombs 4.8 x 10-19 coulombs 6.4 x 10-19 coulombs 8.0 x 10-19 coulombs 9.6 x 10-19 coulombs 1.12 x 10-18 coulombs Calculated charge on oil droplets:

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 10-28 grams