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The purpose of Robert Millikan and Harvey Fletcher's oil-drop experiment (1909) was to measure the electric charge of the electron. They did this by carefully balancing the gravitational and electric forces on tiny charged droplets of oil suspended between two metal electrodes. Knowing the electric field, the charge on the oil droplet could be determined. Repeating the experiment for many droplets, they found that the values measured were always multiples of the same number. They interpreted this as the charge on a single electron: 1.602 × 10−19 coulomb (SI unit for electric charge).