University of San Francisco - USF Don Yearbook (San Francisco, CA)

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THREE ELEMENT YACVl'M TV BE 23 the filament and the grid is positive all are drawn through the grid to the plate. (1 ) If under the same eonditions the field is negative the emitted eleetrons are returned to the eathode. (e) Hut when an alternating E. M. P. is impressed on the grid circuit so that the grid is alternately positive and negative in respect to the filament then the resistance of the input circuit will be variable for the positive half of the cycle and almost infinite for the negative half-cycle. When the alternating current is superimposed on a constant negative grid potential then the resistance of the grid circuit can be said to be infinite. The foregoing explanation contains implicitly the secret of the tube when functioning as a relay. Any variation of the grid potential changes the strength of the field between the filament and the grid causing a like change in the number of electrons passing from the filament to the plate. Po tential variations between the filament and grid occasion variations in the output circuit and, due to the impressed charge on the grid, the power developed in the output is greater than that expended in the input. Enterprising scientists have calculated that about two million billion electrons pass across the grid on their journey to the plate in an ordinary tube under ordinary conditions. The wires in the plate circuit carry approximately two million billion electrons per operating second while seventy million billion arc emitted by the filament per burning second. Although the Vacuum Tube is exhausted to as nearly a perfect vacuum as man can get still it is known that there are remaining in the tube perhaps four hundred and fifty billion molecules of air after the manufacturer has done all in his power to produce a perfect vacuum. Owing to the minuteness of these air particles, which have a radius of nearly ten billionths of an inch and there are so few of them (relatively) that, in a well made tube, the eleetrons, which constitute the space current infrequently, if ever, collide with them. If they do collide ionization by collision takes place which has

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22 T1IE 10 X ATI AX function arc more practical in every sense. The insertion of a grid or third electrode between the filament and the plate of the thermionic valve made it possible to adapt this device to operations that would otherwise have been impossible. The nature of the effect produced by the grid is as follows: Assuming that a space current is flowing between the anode and the cathode of a thermionic valve now if a grid is interposed and a negative charge impressed upon it then it is clear that the flow of ions will be retarded or completely turned back depending on the strength of the charge of the grid. This action is based on a fundamental principle in electricity which holds that like charges repel and unlike attract. For this reason a positive charge on the grid has the effect of drawing the electrons through to the anode. The filament, heated by the current from a battery, can be compared to a gun; continually bombarding the plate with electrons which must pass through the grid to reach the plate. The grid acts as a spasmodic shield; when a current in its positive phase is impressed on the grid, making it positive, the negative electrons are allowed free course to pass through the grid to the plate. When the negative phase of the current possesses the grid and makes it negative, it is as if an impenetrable shield were lowered between the filament and the plate completely stopping the electron barrage. Every time these electrons are allowed to pass from the filament to the plate they produce an impulse in the plate circuit. Through this impulse the wireless wave is detected, relays actuated etc., according to the function required by the specific instance. With respect to the impressed charges on the grid it is well to examine a few demonstrative conditions which are an index to its action under various circumstances. Suppose that the plate remains at a constant positive potential as regards the filament and that the grid is successively (a) positive (b) negative and (c) zero, (a) If there is a copious emission of electrons from the filament and the field between

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Merrick. L, Cronin, J. Neary. J. Healy, J. JUNIOR LAW Deasy. C. Ortega, L. Donovan. I). Riley. F. Slevin. T. (Ilicksbertf. A. Hutthcs, J. McCallion. N.