CATHODES

A cathode is an essential part of a radio tube, because it supplies the electrons necessary for tube operation. In general, heat energy is applied to the cathode to cause it to release electrons. The method of heating the cathode may be used to identify different forms of cathodes. For example, a directly heated cathode, or filament cathode, is a wire heated by the passage of an electric current. An indirectly heated cathode, or heater cathode, consists of a filament (heater) enclosed by and insulated from a closely fitting metal sleeve (cathode) which is coated with electron-emitting material. The cathode is heated by radiation and conduction from the heater.

A filament, or directly heated cathode, can be further classified by identifying the filament or electron-emitting material. The materials in regular use are tungsten, thoriated tungsten, and metals which have been coated with alkaline-earth oxides.

A very important characteristic of any cathode is its electron-emitting ability for a given amount of heat energy. This characteristic called emission efficiency, is the electron space current in amperes per watt of filament or heater power. For convenience, emission efficiency is usually expressed in milliamperes per watt.