Concept of vacuum tube diode with cathode and anode
If the anode potential is reversed, and made negative with respect to the cathode it will repel the electrons. No electrons will be emitted from the anode as it is not hot, and no current flows. This means that current can only flow in one direction. In other words the device only allows current in one direction, blocking it in the other. In view of this effect, the inventor of the diode vacuum tube, Professor Sir Ambrose Fleming called it an "oscillation valve" in view of its one way action.
Although the basic concept of the vacuum tube enabled a rectifier to be made, it does not allow for another form of control of the flow of electrons in the anode circuit. However it was discovered that is a further potential was placed between the cathode and the anode this could be used to control the flow of electrons between the cathode and anode. Once the theoretical idea was devised, it was necessary to implement a way of placing this potential in the right place. A n electrode known as a grid in the form of a thin mesh or wire through which the electrons could pass, was inserted between the cathode and anode. It was found that by varying the potential on the grid, this could alter the flow of electrons. The grid is normally placed at a voltage below that of the cathode so that it repels the electrons and counteracts the effect of the pull on the electrons from the potential on the anode. If the voltage on the grid is varied then it will vary or control the level of current flowing between the cathode and the anode. As such this form of grid is known as a control grid. It makes the vacuum tube into an active device that is capable of amplifying signals.
The basic thermionic tube with three electrodes is called a triode in view of the number of electrodes. To improve the performance of the tube, further grids may be added. These tubes are given generic names that describe the number of electrodes, and therby giving an indication of the type of tube and performance.
The basic concept of the vacuum tube outlined here enables signals to be rectified and amplified. Many refinements have been added in the form of further grids to enable much better performance to be obtained, but the principles involved are all the same.
By Ian Poole