Millikan's oil drop experiment measured the electron's:

1. The droplets initially fell due to gravity, but their downward progress could be slowed or even reversed by an electric field. 2. Balancing gravity against that field gives the charge on a droplet. 3. Since the charge of an electron was now known due to Millikan's research, and the charge-to-mass ratio was already known due to Thomson's research, a simple calculation gave the mass of the electron. 4. So Millikan supplied the charge and Thomson the ratio. _Source: OpenStax Chemistry 2e (CC BY 4.0), Ch 2 'Atoms, Molecules, and Ions', sections 2.1-2.6_