Patent GB-655359-A (GB)
Title: Improvements in or relating to measuring apparatus for the comparison of d.c. potentials or capacities
IPC: G01R19/18
Filed: 1947-12-08
Abstract: 655,359. Measuring D.C. potentials, capacitances. FOXBORO CO. Dec. 8, 1947, No. 32341. Convention date, Dec. 6, 1946. [Class 37] [Also in Group XXXVIII] In measuring apparatus for the comparison of D.C. potentials (which may indicate physical conditions) a vibratory switch synchronously driven from an A.C. supply alternately connects two condensers to be respectively charged in opposite senses from two D.C. sources (e.g. a thermocouple and a standard cell), and to combine their charges at the input of an amplifier to constitute an alternating control voltage dependent in amplitude and phase sense on the magnitude and sense of the difference between the D.C. potentials. The amplifier may comprise a phase sensitive circuit fed by the control signal and the synchronizing supply to differentially energize motor windings to re-adjust one of the condensers until the control voltage is restored to zero, and to operate an indicator or recorder. Fig. 1 shows a temperature recorder in which a thermocouple T is connected to fixed contacts 12 and 13 of a vibratory switch, and a standard cell S is connected to fixed contacts 22, 23 of a further switch. The polarity of the cell is opposite to that of the thermocouple, contacts 13 and 23 are earthed, and the moving contacts 11, 21 are respectively connected through fixed and variable condensers Cf, Cv to the input 51 of an amplifier A of the kind described in Specification 646,981. The switches are vibrated by an armature 58 operated by an A.C. energized coil 62, and successively change the condensers from voltage sources S, T of opposite polarity and combine their charges in the amplifier input to develop an A.C. control voltage dependent on the difference between the voltage sources and therefore upon the thermocouple temperature. The amplifier has a high input impedance, is tuned to the synchronizing A.C. frequency, and comprises a push-pull phase-sensitive vacuum tube circuit energized by the A.C. source and the amplified control signal to energize differentially the windings 70, 72 of the solenoid motor M which is coupled through linkages to operate a recorder R and re-adjust the variable condenser Cv to equalize the charges on the condensers so that the control voltage applied to the amplifier is reduced to zero. The vibratory switch may be replaced by a rotary commutator driven synchronously from the A.C. source while the rebalancing motor may be eliminated, the variable condenser being set manually to determine a required temperature and the control signal may be applied to an indicating instrument operated controller of the kind described in U.S.A. Specification 2,244,564. The polarity of either of the D.C. potentials may be reversed, and the operation of one of the switches inverted when balance is indicated by no change of input voltage occurring when the switch arms change over contacts. In a modification (Fig. 2, not shown), a third capacity is alternately charged from the standard cell and connected to the amplifier input by a third vibratory switch operating inversely to the other switches, the third capacity comprising an adjustable condenser by which the temperature corresponding to the instrument zero may be varied and a temperature variable condenser in proximity to the cold thermocouple junction compensating for temperature changes therein. The thermocouple is connected to the vibratory switch through a filter tuned to the synchronizing frequency and a balanced input network. A plurality of correcting condensers and D.C. potentials may be used with the same measuring instrument. The device is applicable to the comparison of capacities.