Between 1894 and 1897 Perrin was an assistant in physics at the Ecole Normale, during which time he studied cathode rays and x rays, the basis of his doctoral dissertation. At this time, scientists disagreed over the nature of cathode rays emitted by the negative electrode (cathode) in a vacuum tube during an electric discharge. Physicists disagreed among themselves over whether cathode rays were particles--a logical assumption, since they carried a charge--or whether they took the form of waves.
Confirms Nature of Cathode Rays
In 1895 Perrin settled the debate simply and decisively using a cathode-ray discharge tube attached to a larger, empty vessel. When the discharge tube generated cathode rays, the rays passed through a narrow opening into the vessel, and produced fluorescence on the opposite wall. Nearby, an electrometer, which measures voltage, detected a small negative charge. But when Perrin deflected the cathode rays with a magnetic field so they fell on the nearby electrometer, the electrometer recorded a much larger negative charge. This demonstration was enough to prove conclusively that cathode rays carried negative charges and were particles, rather than waves. This work laid the basis of later work by physicist J. J.
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