1. Positive rays

In 1897 J.J. Thomson discovered the electron. The electron is a tiny particle, almost two thousand times lighter than the hydrogen atom, and it is negatively charged.

Thomson referred to electrons as 'corpuscles'. They had previously been known as 'cathode rays', because in a vacuum tube they started at the negative electrical terminal called a cathode and moved away from it. In 1907 Thomson began to investigate the rays that were moving towards the cathode. These rays were positively charged, and Thomson wanted to see if they were also composed of charged particles.

The positive rays had first been observed by Goldstein in 1886. He had cut small holes in the cathode, and seen pencil-like rays streaming through the holes, which he called 'kanalstrahlen', or canal radiation. The positive charge of the rays was not identified until sixteen years later, when Wien deflected them in a magnetic field. They were much harder to deflect than cathode rays and were deflected in the opposite direction, suggesting they could be massive, positively charged particles.

Wien had used electric and magnetic fields to examine the positive rays. He found their maximum specific charge was around two thousand times smaller than that of the electron, suggesting they could be ionised hydrogen atoms.

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