The electron was the first of the sub-atomic particles to be identified. It was discovered by Sir Joseph John Thomson in 1897. His experiments were concerned with the passage of electricity through gases at low pressure. With development of vacuum pumps and the techniques of sealing metal electrodes in glass tubes, made it easier to carry out experiments with gas discharge tubes.

Cathode rays are streams of high velocity electrons emitted from the surface of a metal when a cathode (negative electrode) is heated inside a vacuum tube by thermionic emission.
Thermionic emission is the process of emitting electrons due to heat energy.