J.J. Thomson...?

Question

How did J.J. Thomson come to the conclusion through his cathode ray experiment that electrons, or "corpuscles," have about 1000 times less mass than a hydrogen atom? I know this assumption was incorrect, but how did he arrive at this calculation in the first place?

Answer 1

He bent the beam with a magnetic field and measured the angle of deflection. The ectromagnetic force due to the field depends on the current, and the inertia (proportional to centrifugal force) depends on the mass.