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Electron gun and cathode rays

In the thermionic diode a beam of electrons moves a cross the tube and hits the positive anode. If a hole is made in the anode the electrons will go through it and a stream of electrons will emerge on the other side. This device is called an electron gun.

The anode is made cylindrical as shown in the diagram. A high voltage, often as high as 3000 V, is connected between the cathode and the anode and this accelerates the electrons to a high speed – around 30 000 000 m/s or about 1/10 of the speed of light!



The beam of electrons is called a cathode ray because it starts from the cathode.

1. Cathode rays are beams of fast moving electrons
2. Increasing the heater voltage heats the cathode more and so it gives out more electrons. This makes the beam stronger (brighter)
3. Increasing the anode voltage makes the electrons move faster by giving them more energy
4. Cathode rays do not pass through glass or metal
5. Cathode rays travel in straight lines in a vacuum
6. Cathode rays travel in circles in magnetic fields
7. Cathode rays travel in parabolas in electric fields
8. The range of cathode rays in gases increases as the energy of the electrons increases.

 
 
 
© Keith Gibbs 2011