The "electron gun" or valve diode
Practical Activity for 14-16
The main purpose of this experiment is to explain the principle of an electron gun. You can also use the apparatus to demonstrate a valve diode – a device that lets the current flow in just one direction.
Apparatus and Materials
- Hot filament diode tube and stand
- Power supply, HT
- Power supply, 6.3 V, AC, for the heater filament (this is often included on the HT supply)
- Demonstration meter with a centre zero dial, -2.5 mA. to +2.5 mA
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 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.
Follow the manufacturer’s instructions for setting up the diode.
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 (High Tension) supply to the electrode. Set this to zero.
- The collection plate and its connection terminal in the diode tube.
With no potential difference (p.d.) across the tube, a small current of about 50 mA. flows, owing to the energy with which electrons are emitted from the filament (Edison effect). But this will probably not be noticed in the experiment described here.
- Set up the diode in its stand, and connect the heater filament to the 6.3 V supply.
- Connect the plate in the tube, through the milliammeter, to the HT supply.
- Connect the other terminal of the HT supply to earth and to one of the filament terminals as shown in the diagram below. The supply enables the plate to be at 400 volts either positive or negative relative to the filament.
- With the filament heater switched off, try a big positive potential difference (p.d.) and then a big negative p.d. You could try a bit of drama here by building up the possibility of getting a big current to flow through the vacuum with a big enough p.d.; then feign concern when there is no current.
- In reality, with the filament not glowing, there will be no current for any p.d. (positive or negative).
- Now switch on the filament with a positive voltage on the collection plate. You will get a current.
- Try a negative p.d. on the collection plate. There will be no current.
- This experiment shows that charges can flow through the vacuum - as long as one terminal is heated and that this heated terminal is a cathode.
- It is reasonable to infer that the charges originate from the heated element (because with the heater switched off, there is no current).
- Given that charges only flow through the vacuum when the heated electrode is a cathode, it is also reasonable to infer that the charges are negative. Positive charge would not flow from a cathode to an anode, whereas negative charges will (being attracted to the positive anode).
- You can explain the results of the experiment using the idea of electrons. These tiny negative particles are free to move in the metal. As the metal is heated up, some of them ‘evaporate’ from the surface. They form a ‘gas’ of electrons above the surface of the hot plate. If the heated plate is put in a circuit and made negative with respect to another plate, the electrons are pulled through the vacuum and so a current flows between the plates. If the heated electrode is positive, the negative electrons are pulled back to the electrode’s positive surface. Valve diode
- The essential story is that the diode can carry a current only one way. So you should let the class take the measurements 'both ways' with the valve diode. With the milliameter in the circuit, you can measure the current for different voltages in each direction. You can point out that there is a current when the diode is connected one way round (with the heated element connected to the negative terminal of the supply) and not when it is connected the other way round. In other words, the ‘valve’ lets current through one way, but not the other.
- You could mention that this is the basis of early valve diodes. The birth of the diode marked the beginning of electronics. However, diodes have now been replaced with components made of semi-conducting materials like silicon. Semi-conductor devices are often called ‘solid state’ because they do not rely on the ‘gas’ of electrons passing between contacts in a vacuum tube.
This experiment was safety-tested in May 2007