In 1885, Sir William Crookes carried out a series of investigations into the behaviour of metals heated in a vacuum. The experiment of Crookes and others showed that a heated cathode produced a stream of radiation, which could cause gases at low pressure to glow and which made other substances emit light. The radiation emitted from the cathode was given the name 'Cathode rays'. By mid-nineties it was known that these rays could be deflected by a magnetic field and they carried a negative charge. Some scientists felt that these rays were waves and others were inclined to think they were particles.
In 1897, J. J. Thomson showed that the stream of radiation were indeed particles called electrons. He conducted the famous discharge tube experiment, by passing electricity at high voltage through a gas at low pressure.
A common discharge tube is a long glass tube having two metal plates, sealed at its two ends as electrodes. It has a side tube through which air can be pumped out by using a vacuum pump, so that experiments can be performed at low pressure.