Electron gun

If a wire is heated in a vacuum electrons are "boiled" off the surface of the wire. This wire may also be placed near a plate (the cathode), the plate gets hot and electrons are then emitted from it. If a positively charged plate is put near the wire the electrons will be attracted towards it.

A beam of electrons therefore travels across between the cathode and the anode and this is called a cathode ray. If the plate is bent into a cylinder the electron beam will pass through the centre and out the other side. This is called an electron gun (Figure 1).

If the potential difference between the cathode and the anode is V, the mass of the electron (m) and the charge on the electron (e) the velocity with which the electrons emerge from the electron gun is given by:

Properties of electron beams - cathode rays

These experiments showed that cathode rays were indeed streams of electrons and that the cathode rays had the following properties:
(a) the travelled in straight lines in the absence if fields
(b) they could be deflected by both electric and magnetic fields
(c) they moved in circles in magnetic fields and in parabolas in electric fields
(d) they could cause fluorescence
(e) they possessed kinetic energy which could be changed to heat when they were brought to rest
(f) they could produce X rays if they had sufficiently high energy