Deflecting an electron beam
Practical Activity for 14-16
In this simple demonstration with a fine beam tube you can show an electron beam. You can also bend it using an electric field and a magnetic field produced by some Magnadur magnets. These are interesting in their own right, and are good preparation for other experiments.
Apparatus and Materials
- Fine beam tube and stand
- Power supply, low voltage, variable, 0 - 24 V, smoothed
- Power supply, HT, 0-250 V, with special shrouded connecting leads
- Power supply, 6.3 V, AC, for the heater filament (this is often included on the HT supply)
- Magnadur magnets, 2
- Connecting leads
Health & Safety and Technical Notes
The HT supply can deliver a fatal current. Use 4 mm leads with plugs that have sprung shrouds for all high-voltage connections. Ensure that the member of staff supervising the dark room is aware of the hazards and their control.
The practical work with a HT supplies should only be undertaken by teachers with a good knowledge of HT electricity and the dangers.
Students should observe well away from the apparatus when it is being used.
Post-16 students may undertake the practical with supervision. See Topics in Safety (ASE 2001), Chapter 17...
Setting up the fine beam tube: Follow the manufacturer’s instructions for setting up the fine beam tube. (This demonstration does not involve the Helmholtz coils, so remove these if this can be done simply.)
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 HT supply to the anode. Set this to zero. The negative terminal of the HT goes to a socket, which is often near to the heater terminals.
- The low-voltage supply to the deflecting plates. Set this to zero.
A tube which has not been used for a while may not emit electrons. It may be possible to encourage it to do so by increasing the heater voltage slightly, to around 1 V or so. Monitor it carefully. Ensure that the heater current is only slightly exceeded.
You can use the low voltage power supply or the batteries for deflecting the electron beam.
- A smoothing unit may be needed with the low voltage power supply.
- You will need to ensure that only one point is earthed. The low voltage supply will have the negative terminal earthed. As this is connected to the anode, ensure that the positive terminal of the HT supply is earthed.
- Do not connect the low voltage power supply to the heater. This will damage it. The heater needs a supply of about 6 V. This is usually included on the HT power supply.
- Some power supplies have moving coil voltmeters incorporated in them. This type is helpful in this experiment.
- You can use batteries instead of the low voltage supply. You will need three 6 V battery packs connected together to get a decent deflection. You can change the deflection by increasing the number of cells being used.
- Deflect the beam one way by connecting the negative terminal to the earthed anode and the positive terminal to the deflection plate.
- Deflect it the other way by connecting the positive terminal to the earthed anode and the negative terminal to the deflection plate.
- Select the gun which gives a horizontal electron beam. (There may be a selection switch.)
- Always switch the heater on first, and only when it is glowing turn up the accelerating voltage on the anode.
- When the filament is glowing, carefully increase the anode potential difference (p.d.}. At a p.d. which may be as low as 50 volts, the fine beam should be seen. With some tubes it may take 3-4 minutes to be clearly visible. As the p.d. is slowly increased, the beam will lengthen and strike the glass envelope of the tube.
- Reduce the p.d. and show this transition several times. Do not increase the p.d. beyond about 200 volts.
- With the beam striking the wall, apply 10 to 20 volts (d.c.) to the deflecting plates and observe the movement of the beam. Reverse the connections to the deflecting plates and repeat. Increase the p.d. on the deflecting plates to the maximum available, and repeat the reversing procedure.
- Now deflect the beam with a magnet with face polarity (a Magnadur magnet}. Bring the magnet near to the envelope of the tube and point out the deflection of the beam.
- You can use two such magnets on opposite sides of the tube to produce a more uniform field. See diagram above. Take care not to bring a magnet into violent contact with the glass.
- If there are field coils, then a variable low voltage connected to them (up to 6 V or so) will deflect the beam into a circle, whose radius decreases with increasing voltage.
- This may be students’ first glimpse of an electron beam. Allow them to enjoy it. 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 lifetime.
- The electron beam is visible because there is a low-pressure gas in the tube. Electrons striking the gas molecules give them energy, which is then released as light. When they re-radiate this energy, hydrogen gas glows blue and helium gas glows green.
- Draw the parallel with old television tubes by changing the beam to a horizontal one if possible. This has an electron gun like the fine beam tube. The electron beam is usually deflected magnetically rather than electrostatically and a different method of making it visible is used.
- You could also draw the parallel with a cathode ray tube, as found in an oscilloscope. (See related experiments.)
- You could try connecting a low frequency alternating supply to the deflecting plates. The beam will move from side to side. NB remove any smoothing components first! This is extended in the experiment.
- The hot electron gun releases electrons, and a potential difference of about 180 V will accelerate electrons to about 8 x 10 6 m s -1 . In a TV tube with 25 kV it is about 3 x 10 7 m s -1 .
- Catching up on the catapult field. It is worth reminding students of the catapult effect and Fleming’s left hand motor rule. The deflection of the beam is consistent with the electrons having a negative charge. That is, to explain the direction of the deflection, the current must be flowing into the electron gun. Therefore, the charge on the particles carrying it must be negative, because they are flowing out of the electron gun.
This experiment was safety-tested in May 2007