GB719485A - Improvements in and relating to radiation measuring devices - Google Patents

Abstract

719,485. Radiation measuring. BRITISH THOMSON-HOUSTON CO., Ltd. Feb. 24, 1953 [March 18, 1952], No. 6968/52. Class 40 (3). [Also in Group XXXV] In radiant energy measuring apparatus of the type in which the sensitive member is periodically irradiated for example by means of a copper plate, and in which an alternating voltage is applied to the member, the periodic irradiation being synchronized with the applied voltage, a capacitor is connected in series with the sensitive member so that a direct voltage is developed across the capacitor of magnitude dependent on the radiation intensity. As shown, heat radiation from a source 1 is directed on to a lead sulphide photo-conductive cell 2 through a chopper disc 4 driven by a synchronous motor 5. The A.C. supplying the synchronous motor is also applied across cell 2 and condenser 7 in series the arrangement-being such that the radiation is always interrupted when the voltage across the cell' is of one polarity. A direct voltage is developed across capacitor 7 and it is shown in the Specification that if the fractional change in resistance of the cell is small this voltage is dependent only on the applied A.C. voltage and the fractional change of cell resistance and that if the total resistance of the meter is large compared with that of the cell the meter will be unaffected by small change in cell.resistance, i.e. the apparatus is free from "zero drift." This direct voltage is applied to a meter 11 through a low pass circuit 8, 10. In the arrangement of Fig. 2 the output voltage developed across condenser 7 is fed to the grid of an A.C. fed triode valve 20 the signal developed at the anode, being fed through a smoothing circuit 22, 23, 24 to the grids, of triodes 25, 26 between the anodes of which is connected a winding of transformer 27. The gridcathode potentials of triodes 25, 26 are determined by the signal developed across condenser 7 and determine the impedance of the other winding of transformer 27. Thus, the alternating current through a load 28 is controlled. In the arrangement shown the current increases as the pyrometer output increases and load 28 may be a lamp arranged to irradiate cell 2 when the radiation from source 1 is cut off. The action of the circuit is then to maintain the lamp current so that the two irradiations are nearly the same, the lamp input being an indication of source temperature. Alternatively if the cathodes of triodes 25, 26 are connected to the anode of valve 20, the arrangement operates as an automatic control system to maintain the temperature of the source constant by controlling the current in a heater 28.