Descovery of Electrons

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.J.J. Thomson had balanced the cathode rays between the electric and magnetic forces.

The force (F) on a charged object in an electric field depends on the strength of the electric field (E), multipled by the charge (q) on the object.

F = Eq

The force (F) on a charged object in a magnetic field depends on the strength of the magnetic field (B), multipled by both the charge (q) and velocity (v) of the object.

F = Bqv

Since the forces were balanced:

Eq = Bqv

v = E/B

The velocity was equal to the electric field strength divided by the magnetic field strength. Thomson could measure these field strengths and use them to calculate the velocity of the rays.