In 1897, J.J. Thomson successfully determined the charge-to-mass ratio (^q⁄_m) of the electron. However, because neither the electron's charge nor its mass could be measured independently, both fundamental values remained unknown at the time.

To solve this, American physicist Robert Millikan designed his famous oil drop experiment in 1909 to determine whether electric charge existed in discrete, fundamental quantities. By measuring the motion of microscopic, charged oil droplets suspended between two parallel metal plates under the influence of gravity and an electric field, Millikan proved that electrical charge is quantized—occurring only in integer multiples of a fundamental unit (e ≈ 1.602 × 10^-19 C).

Combining this newly determined elementary charge with Thomson’s charge-to-mass ratio allowed physicists to calculate the mass of the electron (m_e ≈ 9.109 × 10^-31 kg) for the very first time.