The electron gun shoots out a beam of electrons across an evacuated tube. It hits a fluorescent screen placed in its path and when it does the screen glows. If there is no voltage between the two plates the beam will go along the middle of the scale.
If a voltage is applied across the plates with the bottom plate positive, the electron beam will be attracted downwards towards the positive plate showing that the beam really is made up of negative particles. If the top plate is made positive the beam will be attracted upwards. (Figure 2)
A bar magnet can now be held at the side of the tube and you will see that the beam of electrons is deflected up or down depending which way round you hold the magnet. The same thing will happen of course if you use an electromagnet (see Figure 3).