Patent GB-349636-A (GB)
Title: Improvements in and relating to photo-electric cells and circuits therefor
IPC: H01J40/00
Filed: 1930-02-15
Abstract: 349,636. Television ; photoelectric cells. WESTINGHOUSE ELECTRIC & MANUFACTURING CO., East Pittsburg, U.S.A.-(Assignees of Shoup, A. A. ; 841, Holland Avenue, Wilkinsburg, Pennsylvania, U.S.A.) Feb. 15, 1930, No. 5148. Convention date, Feb. 15, 1929. [Classes 39 (i), 40 (iii), and 40 (v).] In a system for use in television or the like, in which carrier-oscillations are modulated in accordance with fluctuations in the illumination of a photoelectric cell, the effect of the capacity of the cell is eliminated in order that the output amplifier may not be overloaded with unmodulated oscillations. This result may be effected by means of a balancing condenser 27, Fig. 1, or of an earthed screen 35 built into the valve, Fig. 3, or by both methods. In the oscillator 1 the plate and grid circuits are coupled at 10, 12, 13, and the carrier oscillations thus generated are supplied to the cathode 21 of a photo-electric cell whose anode 23 is coupled to the amplifier 30 and earthed through a tuned impedance 25, 26 and a biassing battery 24 which prevents the cell from rectifying. In order that the effect of the cell may be that of a pure resistance fluctuating with the illumination and so modulating the carrier, the capacity of the cell is neutralized by a balancing condenser 27, fine adjustment of the balance being afforded by the resistance 14 and switch 20. The amplifier 30 comprises a series of impedancecoupled valves, with indirectly heated cathodes feeding a final power valve which has a directly heated cathode, the required voltages being taken from transformers 8 and 31 and from a potentiometer 5 which is fed with rectified a.c. In a modification, Fig. 2 (not shown), the resistance 14 is omitted and fine adjustment is obtained by the connection of an earthed adjustable condenser to one side or other of the condenser 14. Instead of being balanced out by means of the condenser 27, that fraction of the carrier oscillations which passes through the internal capacity of the electrodes 21, 23 may be shunted through a screening grid 35 which is earthed through part of the battery 24, the remainder of the battery ensuring that all electrons passing the grid shall reach the anode 23. Alternatively the carrier oscillations may be applied to an external grid, Figs. 4, 9, 10 (not shown), or to a control grid inside the photo electric cell, Figs. 5, 11, 12 (not shown), the cathode 21 being earthed, or a screened cathode may be connected to the input of the amplifier 30, the anode being earthed through the battery, Figs. 6, 13, 14 (not shown). Photoelectric cells. A photoelectric cell for modulating carrier current is generally similar to those described in Specification 348,693 and comprises a cathode 21 having a strip 50 bent at an angle; an anode consisting of an axial rod 54 ; a spiral screening-grid 55 supported by a rod 56 with a disc 59 at the top, a wider disc 57 at the bottom, and a lead-extension 63 soldered to the parts 56, 57 ; a metallic sleeve 61 connected to the spiral 55 ; and an external control grid 36. A substance, such as caesium bichromate, yielding a photo-sensitive material, and a getter such as magnesium are deposited at 65 and evaporated by eddy-current heating after the cell has been sealed. The external grid 36 may be omitted, Figs. 7, 8 (not shown), or replaced by ar. additional internal control grid, Figs. 11, 12 (not shown) ; or the grid 55 surrounding the anode may serve as a control grid while a further spira. grid may surround and screen the cathode. The matter described in Specification 279,937, [Class 40 (iv), Telephones &c.], together with that claimed in Specification 308,337, [Class 40 (iii), Telegraphs, Electric], is disclaimed.