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.
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.
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.
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