CHAPTER THREE

Vacuum -Tube Theory

HE science of radio is based upon one and heptode respectively for tubes having two. of the most versatile developments of the three, four, five, six, and seven elements. twentieth century-the electron tube, or as it is more commonly named, the vacuum tube. MECHANICS OF THE It is the utilization of the unique characteris- tics of the vacuum tube in various circuit ar- VACUUM TUBE rangements which makes possible modern radio The original Edison discovery was that a communication; for that matter, long distance heated filament would give off electrons to a wire communication also owes its efficiency to cold plate in the same evacuated chamber. It the versatility of the vacuum tube. was later discovered that if the plate were This chapter is divided into two main sec- charged positively with respect to the filament, tions. The first is devoted to the basic theory a much larger proportion of the emitted elec- of the vacuum tube and to a discussion of the trons would be attracted to the plate. But, if various types of tubes which have been de- the plate were charged negatively with respect veloped up to the present time. The second to the filament, the electron flow would stop. part discusses the application of the vacuum This valve action meant that the vacuum tube tube to the various circuit arrangements which could be used as a rectifier since it would pass have been developed to utilize its character- current only in one direction. It is this rectify- istics. ing action of the diode which is used almost Brief History of the Vacuum Tube. universally for the production of direct current Thomas Edison is credited with the discovery from alternating current as supplied by the a.c. that an additional wire or plate placed inside mains. a lighted incandescent lamp would acquire a Then, the discovery that additional elements negative charge of electricity. J. A. Fleming could be placed between the cathode and plate undertook the study of the Edison Effect in to control the electron flow in any desired man- 1895, and as a result of his findings in 1904 he ner resulted in the simultaneous development patented the two -electrode tube or diode which of the vacuum tube and improvement of the became known as the Fleming valve. Then, in radio art to make use of the greater capabili- 1906, Lee de Forest discovered that a third ele- ties of the improved tubes. In recent years, ment could be placed between the cathode and however, the improvement of the art has com- plate to control the flow of electrons from monly been first, with improved types of one to the other. This third element was called vacuum tubes being developed as the need for a grid from its physical resemblance to the them arose. grid or grate of a stove. The insertion of the Thermionic Emission. The free electrons grid into the space between the cathode and in any metal are continually in motion at all plate in the diode resulted in the most versatile temperatures. But at ordinary atmospheric of vacuum tubes, the triode. temperatures, these electrons do not have suf- In recent times other elements or grids have ficient energy to penetrate the surface of the been added to the original triode to augment material. It is necessary that some form of ex- the electron flow in a particular manner, or to ternal energy be supplied to the surface for give a particular characteristic to the vacuum emission to take place. When this energy sup- tube. These later types have been called multi - ply is in the form of heat, the result is called element tubes. The names for these multi - thermionic emission; when the energy is in the element tubes are obtained by adding the Greek form of light it is called photo-emission. The prefix for the number of elements to the root phenomena of photo -emission is applied in the -ode: diode, triode, tetrode, pentode, hexode, photo -electric tube, while thermionic emission 53

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