Modern Thermionic valves have progressed a long way from the simple diode valve first used by Prof. J. A. Fleming between 1890 and 1896 for detection of high frequency oscillations. Yet all have developed as a consequence of the work carried out by him.
Prior to Prof. Fleming's discovery, work by physicists both in America and Europe had resulted in the discovery that certain substances, particularly metals, had the property of emitting charged particles when heated in a vacuous space. These particles were apparently negatively charged since they could be collected by a positively charged plate of metal but not by a negatively charged one. It is now established, of course, that these particles are electrons.
The essential fact, however, upon which Prof. Fleming fixed was that an evacuated device consisting of an electrically heated wire and of a collector electrode in the form of a plate, would conduct electricity only in one direction, i.e. when the collector electrode was made positive. This, he realised, implied that such a device could be used to convert alternating current into direct current and had particular application to rectifying high frequency oscillations. He was led to experiment in this field and his results were completely successful.
Fleming's work had thus provided an efficient and reliable method of rectifying high frequency waves. As yet, however, no method was known of amplifying small variations in voltage except by means of transformers. In 1907 Lee de Forest conceived the idea of introducing between the heated wire and the collector electrode a mesh of wires. He found that very small variations in potential impressed upon this mesh had the effect of controlling the current flowing to the collector electrode. If this current were made to flow through some form of resistance, potential variations appeared across the resistance in synchronisation with the variation impressed upon the wire mesh and of much larger magnitude. Thus he had achieved a new and convenient method of amplifying small alternating voltages.
These two fundamental discoveries, one by Fleming and the other by De Forest, have been directly responsible for the extraordinary advances which have been made in wireless telegraphy and telephony since 1907. To-day all forms of thermionic valves are fundamentally similar to those used by Fleming or De Forest. They contain a heated electrode known as the " filament " or " cathode," at least one collector electrode known as the " anode," and one or more meshes of wire known as the "grid " or " grids." We shall proceed to discuss these electrodes and their relation to modern valves.
An enormous amount of work has been carried out by physicists and valve engineers on the material used for the filament or cathode.
In almost all Cossor valves the filament consists of a core wire covered with a coating made of a mixture of the oxides of certain of the alkaline earth metals. These oxides, among other peculiarities, have the property of emitting an enormous number of electrons when heated to only a dull red. In addition, these oxides have the further advantage of supplying their emission for an almost unlimited time. Thus it may be seen that on all counts this type of filament is eminently satisfactory for commercial use.
It may not be generally realised that the mass of electrons emitted by a filament may be considerably in excess of the actual mass of the filament coating. As an example, a Cossor Valve having a 2-volt 0.1 amp. filament run at an anode current of 7 m.a. for 20,000 hours represents a passage through the valve of a number of electrons having a total mass of approximately 1-5 milligrams. The total mass of the actual active coating in such a filament is 0-4 milligrams, so that the mass of electrons leaving the filament actually exceeds the total mass of the filament coating. A filament such as is described above is used in all Cossor battery valves. In the case of A.C. mains valves a somewhat different technique is required. Here the source of electrons is heated by A.C. current, and if it is of a filamentary character considerable hum is likely to result in the output from the receiver. Hence an "indirectly heated" cathode is used for these valves. This consists of a hollow nickel tube of circular or flattened section, which is coated on its outside with the usual alkaline earth oxides. A connection is provided to this cathode and it is hsated to a temperature adequate for full emission by means of an insulated wire " hairpin " inside it. The alternating current passes through this wire only, and the insulating material coating the " hairpin " is a good non-conductor even at elevated temperatures. Hence no hum results due to the alternations of the supply being applied to the cathode.
The " collector electrode " now consists of an anode of a more or less complicated design depending upon the type of valve in which it is used. This anode receives all or the bulk of the electrons emitted from the cathode and the " bombarding effect" of this stream of miniature bullets tends to raise its temperature. In consequence the anode in any thermionic valve must be large enough in area to dissipate the heat generated by this bombardment without an undue rise in temperature. The anode is usually in the form of an enveloping box containing the cathode and grids, and in receiving valves it is usually an easy matter to ensure adequate heat dissipation from its surface. In certain cases, carbon deposited on the surface of the anode helps this, and it may be observed that certain Cossor output valves use anodes so treated.
The purpose of a grid, as has been explained, is, in general, to affect or control the flow of electrons from cathode to anode. In addition, however, multi-electrode valves often include grid electrodes, which are maintained at a fixed potential and are used to impart to the valve in question some desired characteristic particularly suited to the use made of the valve. In addition, such an electrode may serve to reduce the capacity between two other electrodes between which it is interposed. Such grids are known as "screens," " accelerating grids " or " suppressor grids." In the succeeding sections descriptions will be given of the constructional details and uses of the many types of valves now manufactured.