 With the mass known, Robert Milliken, a contemporary of Albert Einstein, found a way to measure the charge of an electron. This is the original equipment he used in 1909. The experiment was performed by spraying a mist of oil droplets into a chamber above two metal plates. Some of the oil droplets became electrically charged by friction as they were sprayed through the nozzle. If few droplets would enter the space between the parallel plates, controlling the electric potential across the plates would cause any charged droplets to rise or fall. Finding the voltage that caused a droplet to be suspended above the bottom plate indicates that the downward force of gravity was equal to the upward electrical force. Once Milliken had arduously and meticulously determined the weight of a droplet, he could solve for the charge on the droplet. It was not known how many electrons would attach to each droplet, maybe one, maybe more. So the experiment was carried out a large number of times. The smallest charge found was 1.6 times 10 to the minus 19 coulombs. And all other charges on oil drops were found to be whole number multiples of this one, indicating that it was the charge on a single electron.