Patent GB-319286-A (GB)
Title: Improvements in and relating to light sensitive apparatus
IPC: H01J17/04
Filed: 1929-09-19
Abstract: 319,286. British Thomson-Houston Co., Ltd., (Assignees of Hull, A. W.). Sept. 19, 1928, [Convention date]. Vacuum tubes; low-pressure metallic-vapour apparatus.-A light-responsive system, which may be used for controlling a valve regulating the supply of oil or coal to a furnace, the switching of lamps or signs, or for photometry, consists of a bridge arrangement of which one arm consists of selenium or other light-sensitive material connected between the anode and grid of a thermionic valve, and another arm consists of a balancing resistance connected between the grid and the negative terminals of a source of supply, the remaining arms consisting of the source of supply, an intermediate point of which is connected with the cathode, the arrangement being such that the change in resistance of the selenium &c. device due to a variation in, light causes a change in the potential of the grid, thereby allowing a current to pass through the valve to operate a relay or, in the case of photometry, to produce a reading on an ammeter; the selenium &c. device may be mounted inside a valve of the kind described in Specification 309,442 operating with an arc-like discharge. As shown in Fig. 1, a light-sensitive cell 7 of selenium or molybdenite is connected between a grid 3 and the output circuit. A resistance 8, of a value depending on the resistance of the cell in an activated or inactivated condition, is connected between the grid and the negative side of the supply, and the filament 2 is connected with two, preferably equal, resistances 5, 6 between the supply leads. The grid is normally negatively charged, and activation of the cell by light destroys balance of the bridge and allows a current to pass through the valve and ammeter 9. In a modification two light-sensitive layers are formed on opposite sides of a block of insulating material, and are connected, one between the grid and the anode, and the other between the grid and the cathode. In the arrangement shown in Fig. 3, two layers 30 of selenium are formed in the inner surface of the envelope of a power amplifying valve operating with an arc-like discharge, and having an anode 17, a cathode, and a perforated cup-shaped control electrode 18. The cathode is of multi-cellular form with metal vanes 19 extending between concentric cylinders 20, 21 of nickel or an alloy of tungsten or molybdenum containing thoria. A heating coil 22 is connected at one end with the thickened end of the inner cylinder, and is separated by an insulating lining of magnesia alumina., or thoria; from the inner cylinder. The control electrode is arranged so as to shield the cathode completely from the electrostatic influence of the envelope. The upper ends of the selenium layers are connected with the control electrode, and the lower end of one layer is connected with one end of the secondary 32 of a transformer. The lower end of the other layer is connected with one end of the primary and through the winding of a relay 34 with the anode. In operation the grid normally assumes a negative charge and prevents a discharge. Activation of one of the selenium layers by a lamp or the glow of a self-firing furnace produces a change in the potential of the grid so as to allow a discharge to pass with consequent energization of the relay, which opens a valve regulating the oil or coal supply to the furnace. The filling in the valve may consist of argon at a pressure of 5-20 microns or a drop of mercury may be inserted in the envelope. In a modification the light-sensitive material is coated on an insulating member such as glass mounted in the envelope. In another arrangement a triode controlled by a double selenium cell is coupled through a transformer or resistance with a power amplifying valve.