Maltese cross: casting shadows

Demonstration

This demonstration shows that a metal cross blocks off a beam of electrons. Using a magnet, you can show how the electrons can be deflected, by distorting the shadow they cast.

Apparatus and Materials

• Maltese cross tube and stand
• Power supply, EHT
• Power supply, 6.3 V, AC, for the heater filament (this is often included on the HT supply)
• Bar magnet (optional)
• Old cathode ray TV, if you have one

Health & Safety and Technical Notes

The tubes are fragile (and expensive!) and should be handled carefully. They will implode if broken. Use the stands specifically designed for holding them.

Read our standard health & safety guidance

Set the tube up according to the manufacturer’s instructions.

Ensure that you can identify the following:

• The 6.3 V supply to the cathode heater (if you connect the wrong voltage to the heater you can easily damage the tube beyond repair).
• The EHT supply for the anode. Set this to zero. The cathode is often one of the heater terminals.

If the tube has been unused for some time, the cathode might not emit electrons. Carefully increase the heater voltage by about 1 V, monitoring it. Do not allow the heater current to rise much above the recommended values.

There are sometimes problems with the shadow of the cross turning into a ‘clover leaf’ shape. This can be prevented by connecting the cross to the anode and turning down the high voltage supply a bit.

However, it is not essential to connect the Maltese cross to anything, and it may be more convincing if this is not done.

If the fluorescence disappears, this may be because the cross is getting negatively charged by electrons. The fluorescence will reappear if the cross is momentarily connected to the anode.

With some tubes, electrons hitting the cross cause other electrons to be emitted from the cross. These secondary electrons travel to the positive anode, and keep the cross from becoming appreciably negative with respect to the anode. If the cross is connected to the anode, the cross is at the same potential as the anode all the time. This arrangement may therefore be preferred.

It's worth hanging on to an old cathode ray (CR) TV for the demonstration described in step 6. You might even be able to get hold of cheap or free CR TVs from friends, as people upgrade to LCDs and plasma TVs. They must still be electrically safe and tested each year.

Procedure

1. Set the tube up in the stand. Connect the filament heater to the 6.3V supply. Connect the positive terminal of the EHT power supply to the perforated anode and also to earth. The negative terminal of the supply is connected to the filament.

2. Set the EHT voltage to zero and switch on the 6.3 V supply to the heater filament.
3. With no output from the EHT supply, the light from the filament can be seen on the fluorescent screen at the end of the tube, and there will be a sharp shadow of the Maltese cross.
4. Once the heater is glowing, increase the potential difference (p.d.) to the anode with the EHT. As the p.d. is raised to about 3 kV, the thermionic emission produces fluorescence on the screen.
5. Try bringing a magnet near the tube; the fluorescent shadow will move. The optical shadow will remain undeflected.
6. If you have an old cathode ray (CR) TV, it is entertaining to bring a powerful magnet near the screen or close to the tube. This will distort the picture. It’s advisable not to do this with your best TV. This can cause permanent distortion of some shadow mask colour TVs. It is best to show it on a monochrome TV or old colour TV.

Teaching Notes

• This experiment is best demonstrated to the students in groups of four to five in a darkened room if full value is to be obtained.
• Always reduce the anode to zero volts when not actually observing the beam, because the tube has a finite life time.
• Discuss the fact that the fluorescent shadow only appears when the heater filament is a cathode, showing that the electrons are negative.
• You may be able to reinforce this with the direction of the distortion produced by the shadow. Use Fleming’s left hand rule to show that the electric current is flowing towards the cathode i.e. that negative charges are flowing out of it.
• Point out that the beam does not pass through the metal cross. Therefore the electrons are absorbed by the metal.
• More advanced students may appreciate the fact that the magnet shifts the shadow coherently implies that all particles in the stream are being deflected by the same amount. This means they have the same ratio of charge to mass. All particles are also likely to have the same charge and same mass (though, from this experiment alone we cannot be sure).

This experiment was safety-tested in April 2007