Spark image

Cathode ray deflection tube

It is possible to find out a lot about cathode rays by using a deflection tube like the one drawn in Figure 1.

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.

Beams of electrons (called cathode rays) move in straight lines in a vacuum when there is no electric or magnetic field.

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)

The path of the electrons in an electric field is a parabola.

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).

The path of the electrons in a magnetic field is a circle.

If you increase the anode voltage, then the electrons will be given more energy and so will move faster. They will therefore be more difficult to deflect by electric or magnetic fields.

schoolphysics deflection tube animation

To see an interactive experiment using the deflection tube click on the animation link.

© Keith Gibbs 2013