Fine Beam Tube: a naked oscilloscope
Practical Activity for 14-16
Applying a constant voltage to the deflection plates will move the electron spot. Applying an alternating voltage will produce a line on the screen as the beam moves up and down. You can relate this to the lines produced on a CRO or a cathode ray television screen.
Apparatus and Materials
- Fine beam tube and stand
- 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)
- Power supply, L.T. variable (with AC and DC option)
- Batteries, 12 V, 2 (optional)
- CRO for comparison
Health & Safety and Technical Notes
HT (high tension) power supplies (generally supplying voltages up to 400 V) can cause fatal electric shock.
It is essential that all HT connectors and cables are rated at the voltage to be used. The HT connectors should be the shrouded type so that accidental contact is highly unlikely. Any meter used in the HT circuit should be a type rated for the voltage used, and with shrouded connectors.
All HT connections should be made with the HT switched off, and no adjustments made to the HT connections or wires once the HT is switched on.
The Practical work with 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...
The tubes are fragile (and expensive!) and should be handled carefully. They will implode if broken. Use the stands specifically designed for holding them.
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 High Tension (HT) supply to the anode. Set this to zero. The cathode is often one of the heater terminals.
- The low-voltage supply to the deflecting plates. Set this to zero.
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.
The low voltage power supply should be used with a smoothing capacitor.
Some power supplies have moving-coil voltmeters incorporated in them. This type is helpful in this experiment.
You can use the 12-volt batteries as an alternative to the low voltage supply, especially if your DC supply is not very smooth. The batteries should be connected in series with each other and a centre tap joined to one of the deflecting plates. A lead from the other deflecting plate is connected successively to different tapping points on the batteries, to show the effect of a change in potential difference (p.d.).
- Select the gun which gives a horizontal electron beam. (There may be a selection switch.)
- Heat the cathode, then turn on the HT gun potential difference (p.d.) and let students look at the tube closely in a half-dark room. Allow them time to enjoy looking at the beam.
- Show the beam being deflected when a p.d. of 20 to 30 V is applied to the deflecting plates. If students have seen an oscilloscope in use, point out the comparison. (In this case, you can only deflect the spot on the screen up and down, not horizontally.) You could also point out the similarity to a cathode ray tube as used in many TV sets and computer monitors.
- You can use the smoothed power supply or the batteries.
- Now show the deflection with alternating voltages. The AC output of the L.T. variable-voltage supply should be connected to the deflecting plates and the p.d. slowly increased from 0 to 25 volts. (It is advisable to have one of the deflecting plates connected to the anode.) The spot should turn into a line.
- This may be students' first glimpse of a 'visible' electron beam. Give them time to enjoy looking at it and explaining what is happening. 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.
- 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. Hydrogen gas glows blue and helium gas glows green.
- You could explain the operation of the fine beam tube using a script provided in the Guidance note Electron guns.
- The attraction of the electron beam by the positive plate indicates that electrons are negative. The gun muzzle is also connected to the positive terminal of the HT supply in order to attract the electrons out of the gun.
- Connect the deflecting plates, as shown in the diagram. The electron beam now passes through a transverse electric field produced by the low voltage supply, and is deflected towards the positive plate.
- The higher the potential difference (p.d.) between the plates, the stronger the electric field and the more the beam deflects.
- When the p.d. of the low voltage supply is reversed, the field reverses, and the beam deflects in the opposite direction.
- With an alternating potential difference connected between the deflecting plates, the beam will swing backwards and forwards.
- If the alternation of the potential is rapid, the eye will not be able to follow the beam’s movement. A fan of ‘green light’ will extend from the gun and through the plates, indicating the path of the electrons.
- If students have seen an oscilloscope in use, point out the comparison. (In this case, you can only deflect the spot on the screen up and down, not horizontally.) You could also point out the similarity to a cathode ray tube as used in older TV sets and computer monitors. However, they use magnetic fields to deflect the beam.
- The alternating voltage makes the electron scan out a line (in this case a vertical one). On other TV sets, the electron scans out a horizontal line; then it shifts down and scans another one, and so on 625 times. It looks like a line because of our persistence of vision and the persistence of the phosphorescent screen – i.e. the glow remains on the screen for a very short period after the electron has moved on.
This experiment was safety-tested in April 2007