An experiment begun in 1909 by Robert Millikan to determine the charge on an electron. Between two horizontal oppositely charged plates Millikan introduced a fine spray of oil droplets. He first measured the mass of the oil drops by measuring their rate of fall under the influence of gravity and against the air resistance. Then using X-rays to ionize the air he noted the changed rate of fall of droplets that were attracted to the lower plate after these droplets had acquired a charge from the ionized air. These figures enabled Millikan to conclude that the acquired charge was always a simple multiple of a basic unit. He found this basic unit to be 4.774 × 10−10 esu, which was the value for the charge on an electron used until it was improved on in 1928.