Patent GB-629478-A (GB)
Title: Improvements relating to electrical measuring equipment
IPC: G01R17/10
Filed: 1947-01-27
Abstract: 629,478. Resistance measurements. AIRSPEED, Ltd., CANDELAND, H., and CLARKE, W. H. V. Jan. 27, 1947, No. 2502. [Class 37] [Also in Group XXXVIII] An automatically balanced D.C. Wheatstone bridge network (Fig. 1) comprises standard resistances 6, 7, a balancing potentiometer 8, a resistance strain gauge 2, a dummy compensating strain gauge 3, and a balance preset potentiometer 4. One or more alternative branches such as 2a, 3a, 4a are selectable by a switch 5 which may operate cyclically. The contact arm'of potentiometer 8 is continuously rotated with pulleys 14, 15, carrying an insulating band 16 having metal styluses 17, which cyclically traverse the width of electrically sensitive recording paper 11 driven from roller 12 to roller 13, across a conducting backing plate 19. Contact between a lateral conducting strip 18 and the sensitive paper is made by one or other of the styluses. As the arm of potentiometer 8 rotates, a D.C. signal varying from a negative value through zero to a positive value is fed from the bridge diagonal to the input of a D.C. amplifier (Fig. 2) wherein the seriesconnected triodes 20, 21, supplied with anode current through a cathode follower stabilizer triode 22 from a stabilized supply, operates as a balanced D.C. amplifier in which the triodes 20, 21 are one branch of a bridge network of which the resistances 31, 32 are the other branch. The output between the joint of the resistances and the anode of triode 20 is fed to the triode 33, cathode coupled to the pentode 36 which is resistance coupled to the cathode follower triode 39. The voltage across the cathode resistance 41 is coupled to the grid of a tetrode cathode follower 42 through a long time constant circuit 46, 47 which is cyclically shorted by switch 47a when the potentiometer arm returns from the end to the beginning of its travel. The signal across the variable cathode load 45 changes with the arm setting of the potentiometer 8 about a datum preset by the cathode load to a value such that the trigger circuit comprising double triode 49, Fig. 3, which has two alternative stable conditions, just operates when the bridge is balanced at zero strain. The output of the trigger circuit is differentiated by condenser 58 and resistor 59, giving a sharply peaked signal at the instant of bridge balance, which is amplified by triode 60, sharpened by the positively biassed triode 67 and fed to the cathode follower tetrode 71 which is coupled through a coaxial lead to supply a positive going signal pulse to the normally cut-off tetrode 83, Fig. 1, which then discharges condenser 90 through the recording paper from the stylus to the back plate, marking the paper according to the balance position of the potentiometer and the instantaneous strain value. Cyclically repeated, the marks form a graph of strain variations against the recording paper movement. The Provisional Specification refers to a device wherein the balance potentiometer slider is not connected to the valve circuit while at rest, its movement being reciprocating and the signal phase being automatically reversed for the reverse direction.