Patent GB-399641-A (GB)
Title: Improvements relating to automatic potentiometers and like balancing arrangements and applications thereof
IPC: G01R17/02
Filed: 1932-08-25
Abstract: 399,641. Current and pressure meters; potentiometers ; resistance measurements. PROSSER, L. E., National Physical Laboratory, Teddington, Middlesex. Aug. 25, 1932, No. 23812. [Class 37.] In an automatic potentiometer or like balancing arrangement any current or voltage representing out of balance automatically varies the current in a mechanically unchanged balancing or potentiometer circuit until balance is restored. As shown in Fig. 1, the voltage e to be measured is balanced against the potential drop across a standard resistance R in the anode circuit of a valve V. Lack of balance operates a reflecting astatic galvanometer G in one direction or the other so as to illuminate one or other of two light-sensitive cells PI, P2 which control the grid potential of the valve V, rapid changes of potential being prevented by a condenser C. The anode current is thus automatically varied until balance is obtained, and is measured on a milliammeter A, the voltage e being calculated from the known value of the resistance R. Stops are fitted to limit the travel of the galvanometer mirror so that the light beam is incapable of falling clear of both cells P1, P2. One of the cells P1, P2 may be replaced by a resistance. In a modification, Fig. 2, the galvanometer and lightsensitive cells are replaced by a polarized relay M, which controls the current through resist'- ances L1, L2 and thereby the grid potential of the valve V. The galvanometer G, Fig. 1, may be replaced by an electrometer, opposite quadrants of which are. connected together. The voltage e may be compared with a second voltage of approximately equal value tapped off from a normal potentiometer, the resistance R forming part of the potentiometer circuit. This arrangement may be employed for measuring deviations of temperature in a thermostatically controlled furnace. A relay controlling a portion of the heating current of the furnace mav be inserted in the anode circuit. By using one or more large valves or thyrotrons in parallel the anode currents may be used directly for furnace heating. Fig. 5 shows a modification for recording' small changes of resistance. The varying resistance R is balanced in a Kelvin bridge K, the galvanometer G of which controls the light-sensitive cells P1, P2. The anode current of the valve V passes through a portion p of the standard resistance P of the bridge. A small change in the resistance R upsets the balance of the anode current flowing through the part p of the bridge, which is restored by a change in the resistance P. This current is measured on the milliammeter A and is proportional to the change in the resistance R. In the construction shown in Fig. 6, a light-sensitive cell P is connected in the potentiometer circuit, the current in which is directly increased or decreased by the beam of light from the galvanometer G. The cell P mav be of the selenium type, and may be shunted by a resistance Sh to obtain greater current. Several cells may be employed, all being affected by the light beam. One or more light-sensitive cells may be employed for direct variation of the resistance of the balancing circuit, or reflected heat may be employed to vary the resistance thermally.