Patent GB-696512-A (GB)
Title: A method of comparing inductances or capacitances and apparatus therefor
IPC: G01R27/02
Filed: 1951-03-08
Abstract: 696,512. Electric control systems. GLADSTONE, R., and WIGGINS TEAPE & Co. (1919), Ltd. May 2, 1952 [March 8, 1951], No. 5649/51. Class 40 (i) [Also in Group XXXVI] Each of two arms of a balanced circuit comprises one of two nearly identical impedances and an A.C. or pulsating voltage is applied to the circuit whereby the phase changing effects of the impedances can be utilized to trigger the circuit and, except when the impedances are identical, one arm of the circuit will be triggered first. The two impedances H1, H2, Fig. 1, to be compared comprise two inductances with U-shaped laminated cores. One inductance HI is mounted rigidly in a frame which is free to rise and fall and rock sideways above an iron roller 1 on which a continuous web of paper runs in a paper making machine where it is desired to control the thickness of the paper. The other inductance H2 has an armature 2 of high permeability across its poles which is adjustable by a micrometer screw 3 driven through gearing 6 by a split field motor 7, the armature of which is continuously excited by D.C. A.C. power from mains 8 is supplied through a switch 9 to an interference suppressor 10 connected to a heater transformer 11, rectifier 12, and and unit 17 containing amplifying valves and thyratrons which supply D.C. for energizing the field windings of the motor 7. The suppressor 10 is also connected to a rectifier 13 which supplies current to the armature of the motor 7, and to an electromagnet 14 the circuit of which is periodically completed by a cam 15 rotatable with the paper making machine to close a switch 16. A recording device comprises a stylus 20 marking on a travelling paper band 19 driven through a pawl 21 and ratchet-wheel 22, the pawl being operated by the magnet 14. The stylus 20 is moved across the paper band 19 by a screw 25 and gearing 26 driven from the gearing 6. Switches 27 limit the lateral movement of the stylus 20. As shown' on Fig. 2, the inductances H1, H2 form two arms of a bridge, the other two arms of which are formed by resistances R1 and a balancing potentiometer P, through which an A.C. potential to earth is supplied. Across the other diagonal of the bridge are connected the grids of valves VI, V1<SP>1</SP> through high resistances R2 which quench the anode currents during the negative half cycles on the bridge. Matched capacitors C1 adjust the phase of the unquenching point. The valves VI, V1<SP>1</SP> are coupled through valves V2, V2', V3, V3' and stopper resistances R6 to the grids of two thyratrons V4, V5, which are so biased that they will not fire unless a pulse is delivered from V2, V2<SP>1</SP>. If H1 has a greater inductance than H2, V1 1 becomes conductive before V1<SP>1</SP> and V4 will fire before V5. Similarly if H2 is greater than H1, V5 will fire before V4. When either thyratron fires, the cathode current flows in the common cathode resistor R12 and a positive potential is applied through a resistor R11 to the grids of the valves V3, V3' which become conducting and prevent delivery of further signals from the valves, V2, V2<SP>1</SP>. The anode currents of the thyratrons V4, V5 energize the field windings F1, F2 of the motor 7, which adjusts the inductance H2 to equality with H1. The gap in the magnetic circuit of the inductance H2 is shown on the indicator 18, which is also an indication of the gap in the magnetic circuit of the inductor H1. Variable condensers may be substituted for. the inductances H1, H2.