GB243349A - Improvements in or relating to electrical means for transmitting pictures, light fluctuations and the like - Google Patents

Abstract

243,349. Marconi's Wireless Telegraph Co., Ltd., (Assignees of Ranger, R. H.). Nov. 18, 1924. [Convention date]. Thermionic amplifiers. -In an arrangement for translating light fluctuations into corresponding electrical impulses as in copying telegraphy by the use of a photo-electric cell, the cell is caused to operate on the straight portion of its characteristic bv subjecting it to light of constant intensity in addition to the fluctuations, and means are provided for amplifying that portion of the output current from the cell which is due to the fluctuations. The cell p. Fig. 1, mounted in a light-tight casing H is subjected to illumination of constant intensity from a. source S<2> within the casing and to varying illumination through a. lens or opening L from successive small areas of the picture carried bv a drum D. The cell is in circuit with a battery 2 and a high resistance 3 of which one end is connected to the filaments of similar thermionic amplifying- valves 6, 6<1> and the other end through a bias battery 4 to the grid of the valve 6. The grid of the' valve 6<1> is connected through a bias battery 5 to the common filament lead, and the plates of tho valves 6, 6<1> are connected through similar high resistances 8, 9 respectively to the positive terminal of the common plate current source. The plate of valve 6 is also connected through the primary 14 of an audio frequency transformer to the plate of a third valve 10, and the plate of valve 6<1> is connected directly to the filament of valve 10. The grid circuit of this valve is provided with an oscillator 11 of audio or other readily amplifiable frequency. The secondary 15 of the transformer is connected across an amplifier 16 of which the out'put circuit includes a rectifier 18 and resistance 17. The whole forms a Wheatstone bridge arrangement of which the four arms are constituted by (1) plate circuit of valve 6 between a and b, (2) plate circuit of valve 6<1> between a and d, (3) resistance 8, (4) resistance 9. With the cell p illuminated only by the source S<2> adjusted so that the cell is on the lowest part B, Fig. 2, of the straight portion B, C of its characteristic, the bridge is balanced by adjustment of the batteries 4, 5, and any subsequent unbalancing due to illuminaltion from the picture results in flow of unidirectional pulsating current between band d through the transformer primary 14 and valve 10. This current produces audio-frequency impulses in the secondary 15 giving rise to direct current across the resistance 17.