Which experiment first measured the electron’s charge-to-mass ratio?

J. J. Thomson’s cathode-ray experiment first measured the electron’s charge-to-mass ratio.

In experiments published in 1897, Thomson deflected cathode rays with electric and magnetic fields and compared the resulting paths. The measurements showed that the rays consisted of particles with a very large charge-to-mass ratio compared with ions, supporting the identification of a subatomic particle later called the electron.

Thomson’s work established that atoms were not indivisible and helped identify the electron as a universal constituent of matter. He measured the ratio e/m rather than the electron’s charge or mass separately.

Robert Millikan’s oil-drop experiment later measured the electron’s elementary charge. Combining Millikan’s charge with Thomson’s ratio allowed the electron’s mass to be calculated.