Spark image

Maltese cross tube

This is an evacuated tube that can be used to show some of the properties of a beam of electrons (cathode ray).

The electron gun fires a beam of electrons that spread out as they travel across through the vacuum towards the right hand side of the tube. In the path of the beam there is a metal cross. The electrons that hit the cross are stopped by the metal, but those that get past it hit a fluorescent screen at the far side of the tube which glows green when the electrons collide with it (See Figure 2).

A sharp silhouette of the cross can be seen on the screen and this shows that the electrons travel in straight lines.

The anode voltage is a few thousand volts and the same voltage is also connected to the maltese cross. If this voltage is increased the electrons gain more energy and so are moving faster when they hit the screen. This makes it glow more brightly. Turning up the heater voltage also makes the screen brighter because the electron gun is producing more electrons due to the hotter heater.

If you touch the front of the screen the silhouette will move because you are earthing it. Electrons that had collected on the glass flow through you to the ground.

Holding a magnet near the tube (especially near the electron gun or near the silhouette of the cross) will distort the picture showing that electron beams are affected by magnetic fields.

© Keith Gibbs 2011