Patent GB-592241-A (GB)
Title: Improved means for the detection and measurement of magnetic fields
IPC: G01R33/05
Filed: 1945-01-08
Abstract: 592,241. Magnetic tests and measurements. HUGHES & SON, Ltd., H., SPROULE, D. O., and HUGHES, A. J. Jan 8, 1945, No. 659. [Class 37] Two coils are arranged one inside the other with their axes perpendicular and a thin sheet of high permeability ferromagnetic material is disposed in both coils ; alternating current is supplied to one coil and a current is induced in the other coil by a magnetic field component parallel with its axis, the induced current being measured by phase-sensitive means. As shown, in Fig. 1, a magnetizing coil a is wound about an axis parallel to the co-ordinate axis O, Y, and a pick-up coil b is wound about an axis parallel to the co-ordinate axis O, X, and is enclosed within the coil a. A thin flat sheet c of ferromagnetic nickel iron alloy is arranged within both coils a, b. When the coils are placed in a steady magnetic field in the direction O, X, parallel with the axis of the pick-up coil b and the coil a is fed with A.C. from a source d so that the permeability of the nickel iron alloy in the direction O, X is reduced nearly to unity, the coil b has induced in it a voltage which is a function of the intensity of the magnetic component parallel to the axis of the coil b. When the steady magnetic field is in the direction O, Y, i.e. parallel with the axis of the magnetizing coil a, the output of the coil b is zero. The pick-up coil b is applied through an amplifier to a phasesensitive rectifier. The frequency of the output being double that of the source d, a phase reference system in shunt from the source d is fed through a frequency doubler to the phasesensitive rectifier, the output from which is measured by a centre-zero galvanometer. A reaction coil may be arranged coaxial with the pick-up coil b and in series with the galvanometer so as to generate a magnetic field in opposition to the component of the magnetic field being measured. In a modification, Fig. 4, the magnetically responsive unit consists of a tube g with a thin wall h of high permeability nickel iron alloy, a toroidal magnetizing coil a, and a pick-up coil b wound in the form of a solenoid across the turns of the coil a. Two such tubes may be arranged with their axes parallel and their pick-up coils in series, a single magnetizing coil being wound through the bores of both tubes. As shown in Fig. 7, two magnetically responsive units a, b, c and a1, b1, c1 are arranged with the axes of the pick-up coils b, b1 at right angles to one another in the same plane. The magnetizing coils a, a1 are connected in series with an A.C. source d, and the outputs of the pick-up coils b, b1 are fed to phase-sensitive amplifiers l12 the sine and cosine currents produced by which are converted to potentials by being passed through resistances and supplied to the two pairs of plates p, n of a cathode-ray oscillograph q. The deflection of the spot from its central position indicates the magnitude and direction of the field. In another arrangement the sine and cosine currents are fed to a selfsynchronized remote indicator with a polarised rotor r, Fig. 8, a coil n1 in series with the reaction coil o, and a coil p1 in series with the reaction coil o1. In a modification the rotor r is replaced by a movable coil fed from a frequency doubler.