Millikan worked on an experiment in which he measured the charge on a single electron. Thomson had already discovered the charge-to-mass ratio of the electron. The actual charge and mass values were unknown. Therefore, if one of these two values were to be discovered, the other could easily be calculated. "His experiment measured the force on tiny charged droplets of oil suspended against gravity between two metal electrodes. Knowing the electric field, the charge on the droplet could be determined."