Patent GB-562735-A (GB)
Title: Improvements relating to measurements of magnetic fields
IPC: G01R33/04
Filed: 1943-03-29
Abstract: 562,735. Magnetic tests and measurements. BRITISH ELECTRICAL & ALLIED INDUSTRIES RESEARCH ASSOCIATION, BUTTERWORTH; A., and TURNEY, A. March 29, 1943, No. 5064. [Class 37] [Also in Group XXXVII] A recording magnetometer comprises an instrument with a deflecting element, the direction of deflection of which is independent of the direction of the magnetic field being investigated, in which an auxiliary magnetic field which is periodically reversed is applied to the sensitive element of the magnetometer so as to yield readings both when the auxiliary field is assisting and when it is opposing the field to be measured, in order that the difference between the two sets of readings may afford a measure of the change in field being investigated, unaffected by any change in the zero of the instrument. The bridge shown in Fig. 1 has in adjacent arms AD, DB a pair of sensitive elements consisting of parallel wires 7, 8 of an alloy whose resistance varies with the magnitude of a continuous magnetic field applied parallel to the axis of the wires. These are connected in the arms of the bridge through transformers 9, 10 so as to convert the low impedance of the wires 7, 8 to a higher value. In series with each of the wires 7, 8 is a loop of copper wire 11, 12, each loop being located close to the alloy wire to which it is not connected and of a resistance such that the resistance balance of the bridge is not affected by equal changes in temperature of the wire 7 or 8 and its corresponding copper loop 12 or 11. The arm AD includes a variable resistance 5 and a variable reactance 6 comprising a condenser bridged across a variable portion of a resistance. The arms AC, CB consist of equal resistances 3, 4. The bridge is supplied with A.C. from a valve oscillator having a frequency of about 1000 c.p.s. The detector terminals of the bridge are connected to an amplifier and rectifier 13, the output of which operates an electromagnetic recorder 14. This has a drum driven at 4 revolutions per hour and carrying a record chart on which marks are made at predetermined time intervals. The wires 7, 8 are mounted parallel to each other and each is surrounded by a coaxial coil 15, 16. These are connected in series and are supplied with D.C. from a potential-dividing resistance 17 through a milliammeter 19 and a reversing switch 20. The coils 15, 16 are reversely connected so as to apply equal and opposite magnetic fields to the wires 7, 8. The reversing switch 20 is operated through a relay 21 by an interrupter incorporated in the clockwork mechanism of the recorder 14 and shown in Fig. 2. On. the shaft 37 of the recorder drum is a toothed wheel 38 making 4 revolutions per hour. This acts as a cam-wheel with 15 teeth, each tooth operating the striker of the recorder. From the face of the wheel 38 project five pairs of pins 39, the pairs being separated by gaps corresponding to one tooth. The pins successively engage a pair of blade contacts 41 connected to the terminals 43, Fig. 1, and thus operate the relay 21. Between each pair of pins the contacts 41 escape and a block of insulating material 44 rests on the next pin 39a leaving the circuit of the relay 21 open for the next period. The result is that there are twice as many recorded points in the trace corresponding to one direction of field set up by the coils 15, 16 as there are in the trace corresponding to the other direction, the traces thus being easily distinguishable. The separation of the two traces at right angles to the time axis is a measure of changes of the field being investigated. If a steady. field of known value is applied to both wires 7, 8, the instrument may be calibrated. The component of the field being investigated in the direction of the axis of the sensitive element may be reduced to zero under initial or normal conditions by the application of a steady control field to the sensifive element.