Patent GB-532333-A (GB)
Title: Improvements in and relating to magnetic surveying apparatus
IPC: G01R33/06
Filed: 1939-07-25
Abstract: 532,333. Magnetic tests ; resistance measurements ; resistances. HARRISON, E. P. July 25, 1939, No. 21557. [Class 37] A magnetic surveying apparatus in which the change in impedance of a wire of high permeability nickel iron alloy is employed to detect a change in the external magnetic field is provided with two such similar wires arranged as two arms in a balanced A.C. bridge network and means are provided for determining the correction necessary to restore the balance when the impedance of one of the wires changes due to a change in the external magnetic field. As shown diagrammatically in Fig. 1, the two high permeability wires a, b are arranged to form two arms of a bridge, the third arm of which comprises a fixed resistance e formed of two wires e1, e2 of materials such as platinum and brass, tin and manganin, or aluminium and brass so that the resistance shall have the same temperature co-efficient and inductance as the wires a, b. The wires e1 e2 are insulated from one another and soldered or welded together at their ends to form a parallel combination, which is then wound on a grooved cylindrical bobbin. The fourth arm comprises a variable inductance c in series with a variable non-inductance resistance d. The wires a, b may be about 9 inches long and 0.0175 inch diameter and composed of an alloy of 16.6 per cent. of iron and 77.3 per cent. of nickel together with 5 per cent. of copper and small percentages of manganese, chromium, silicon, and other elements to give the high permeability characteristic. Each wire is enclosed in a tube a<1>, Fig. 2, of insulating material, such as paper impregnated with artificial resin, and is anchored to a metal terminal a<2> at its upper end whilst its lower end is connected to a spring b<1> anchored to a metal terminal b<2>. The tube a<1> is filled with petroleum jelly or other viscous material and one or more solenoids are wound on the exterior of the tube for polarizing and compensating purposes. For measuring the vertical component of the earth's magnetic field, the wires a, b have to be arranged vertically. To effect this the terminal a<2> includes a suspension a<3> which may be an articulated universal joint, a ball and socket, a loop and hook with or without knife edges, or a metal filament, oscillation being damped by vanes b<3> mounted on the lower terminal b<2>, immersed in oil or other liquid b<4>. The bridge network shown in Fig. 1 is divided to form two instruments A, B, Fig. 3, connected by a three-core cable. If the bridge is balanced with the instruments A, B close together and the instrument B is then removed to a distant position the balance is upset if the earth's field is different in the two positions and the value of the impedance correction will be a measure of the field intensity difference. The wires a, b are polarized by windings a<4>, b6 so as to utilize a sensitive part of the field resistance curve. The winding b<6> is overwound with a compensating coil b<9> in series with a milliammeter b<10>, variable resistance b<11>, and D.C. source b<12>. The resistance b<11> is adjusted to balance the bridge again, the reading on the milliammeter b<10> then being a measure of the change of magnetic field. The alternating current source f comprises a thermionic oscillator having a frequency of about 500. When the horizontal component of the earth's field is to be measured the wires a, b are mounted horizontally.