The Discovery of the Electron

In the late nineteenth century some physicists were busy in studying the phenomenon of conduction of electricity through gases at low pressure in a discharge tube (A common discharge tube consists of a glass tube with two metal plates sealed at its two ends, and a device for pumping out the air present in the tube). One of these experiments led to the discovery of electrons. When a source of high voltage electricity of the order of 10,000 volts was connected to the electrodes, no electricity flowed between them. If, however, some of the air was removed from the tube, electricity began to flow from one electrode to the other through the air in the partially evacuated discharge tube and the air in the tube gave off yellowish red light. When still more air was evacuated and the pressure was decreased to say nearly10–4 mm of mercury, the light emitted by the air faded and the walls of the glass tube glowed with green fluorescence due to radiation traveling from cathode to anode in straight lines. Since the rays were coming out from the negative electrode, i.e., cathode, these were called Cathode Rays. Soon after the discovery of the cathode rays, their properties were studied by a number of workers, J.Perrin showed the rays to be streams of negatively charged particles.

1. Cathode rays

J.J. Thomson became the third Cavendish Professor of Experimental Physics in 1884. One of the phenomena he studied was the conduction of electricity through gases.

One subject which interested Thomson was cathode rays. These rays are emitted at the cathode, or negative terminal, in a discharge tube. In 1879 Crookes had proposed that the cathode rays were 'radiant matter', negatively charged particles that were repelled from the negatively charged cathode and attracted to the positively charged anode.

The nature of the cathode rays was controversial. Although Thomson thought the rays must be particles, many Europeans thought they were an 'etherial disturbance', like light. In Germany Hertz had observed the rays passing through thin sheets of gold. It seemed impossible that particles could pass through solid matter.

Hertz had also found (wrongly) that the rays were not deflected by electric fields. In 1897 Thomson repeated Hertz's experiment.

2. J.J.'s experiment

When he repeated the experiment Thomson removed more of the gas from his tube thus producing a much better vacuum. When he passed the cathode rays through an electric field a deflection did occur.

The cathode rays moved towards the positively charged plate so the rays must be negatively charged.
The gas in Hertz's tube had become ionised by the electric field produced by the plates. The positive ions moved towards the negative plate and the negative ions towards the positive plate. The net charge on the plates was therefore reduced and so the strength of the electric field was reduced. The remaining electric field deflected the cathode rays very little or not at all.

J.J. Thomson realised that he could deflect the cathode rays in an electric field produced by a pair of metal plates. One of the plates was negatively charged and repelled the cathode rays, while the other was positively charged and attracted them. Hertz had not been able to do this because he had left too much gas in his discharge tube.