In 1909, Robert Millikan working at the University of Chicago succeeded in measuring the charge on the electron, he allowed a fine spray of oil to settle through a hole into a chamber where he could observe their fall. The top and bottom of the chamber consisted of electrically charged plates. He introduces the source of x-rays which can cause creation of charges when they strike matter. Charges produced by the x-rays attached to an oil droplet producing one or more charges on the droplet; when there is no voltage applied, the fall of the droplets is determined by their mass and the viscosity of air through which they fall. When a voltage is applied, the droplets that have a negative charge will fall more slowly, stop falling, or even rise depending on the number of charges on them. By adjusting the applied voltage and observing the droplets both with voltage off and voltage on, Millikan was able to determine that the charges on the droplets were all multiples of a smallest , 1.6 x 10 to the -19 coulombs. He took this to be the charge on a single electron.