Electron deflection tube: using an electric field

Demonstration

The deflection tube allows you to show the parabolic path of an electron beam passing through a uniform electric field. The graduated scale allows you to take measurements if you wish. This is the main advantage of the deflection tube over the fine beam tube.

Most of the qualitative ideas of this experiment can be shown using the experiment...

Deflecting an electron beam

Apparatus and Materials

• Power supply, EHT, 1 (or 2 if a second one is available)
• Power supply, 6.3 V, AC, for the heater filament (this is often included on the HT supply)
• Magnadur magnets, 2 (optional)
• Electron deflection tube and stand

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.
• The terminals for the deflecting plates.

The basic deflection is achieved by bending the beam with the same voltage as is used to accelerate it. With this simple arrangement, it is not possible to show the effect of varying the deflecting potential difference (p.d.) on its own, because the anode p.d. would also be changed. Changing the accelerating p.d. alters the speed of the electrons and so leaves the deflection unaltered.

If two EHT power supplies are available, you can use the following arrangement to produce a variable deflection.

It is very important to earth the anode in this case. If the cathode were earthed, for example, there could be 10 kV between the positive terminal of the second power supply, and the neutral side of its mains winding. This is likely to damage the insulation of the transformer.

With the tube anode at earth potential, the heater circuit will be 5 kV below earth potential, and therefore the heater circuit connectors should be made so that accidental contact with the circuit is highly unlikely. The connectors and cables should be rated at better than 5 kV. Use a 6.3 V AC supply designed for valve heater circuits, and ensure the transformer isolation is rated to withstand 6 kV across the secondary and primary winding, and secondary winding to earth. Avoid the use of batteries or general power supplies for the heater circuit.

The deflecting power supply can also be connected the other way round, to make the deflecting plate negative to the anode.

The beam from the deflection tube is produced by a horizontal slit in the anode. So the beam fans out to produce a ‘V’ of electrons in the horizontal plane. This is aimed at a vertical fluorescent screen inside the tube. The vertical screen is at an angle to the beam direction. So the fan of electrons cuts across the screen, producing a straight line along it.

Procedure

1. Set up the deflection tube in its special stand.

2. Connect the 6.3 V supply to the filament. Make sure you connect the 6.3 V supply to the filament. (See technical note 2 above.)

3. Start with the deflection plates connected together and also connected to the anode on the tube.
4. Connect the negative terminal of the EHT supply to the filament and the positive terminal to the anode.
5. Set the EHT to zero volts, and switch on the 6.3 V supply to the heater filament.
6. With no output from the EHT supply, the light from the filament produces a line on the inclined fluorescent screen where the light strikes it.
7. Increase the potential difference (p.d.) to about 3 kV: a fluorescent line appears. This is the path of the electron beam. Point out that the electron beam travels in a straight horizontal line.

8. Then, while one plate is left connected to the anode, connect the other plate to the negative terminal of the EHT supply. This produces a vertical electric field between the plates, deflecting the beam into a parabolic path.
9. If you have not shown an electron beam being deflected by magnets, you could do it here. See here...

   Deflecting an electron beam

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.
• The beam is deflected, which shows there is a force on it. The force is consistent with the beam being made of negatively charged particles.
• The beam is deflected by a finite amount. So it must be made of something with mass. This seems obvious now, but, it is an important piece of deduction. We can deduce that the beam is made of particles with some mass and a negative charge.
• The beam stays intact as it is deflected. At first glance, this suggests that all the particles are the same. However, the mathematics shows that the shape of the curve is independent of the charge and mass of the particles. This is because, if the charge increases, the acceleration will increase in both the electron gun and between the deflection plates.
Likewise, any changes in mass will produce the same proportional change in acceleration in both the electron gun and the deflecting field. See guidance note:

Deflection in electric fields

• The beam travels at a uniform horizontal velocity and so the horizontal displacement varies linearly with time. It also experiences a constant vertical force, so it has a constant vertical acceleration, a. The vertical displacement, s_v , varies as the square of time, t . (s
_v =0.5 _at_ ² ). Hence the path of the beam is a parabola. See guidance note:

Deflection in electric fields

• The fluorescent screen has a graticule on it, and the shape of the parabolic path for different accelerating voltages can be recorded.
• This is analogous to a ballistic experiment in a uniform gravitational field. Whenever you throw something on the surface of the Earth, it traces out a parabola because the vertical acceleration is constant and the horizontal velocity is constant.

This experiment was safety-tested in April 2007