GB575463A - Improvements in and relating to means for the measurement of magnetic field intensities - Google Patents

Abstract

575,463. Magnetic measurements. HUGHES & SON, Ltd., H., SMITH, E., and HUGHES, A. J. March 18, 1944, No. 5064. [Class 37] A square alternating current bridge network having arms composed of nickel iron alloy wires, as described in Specification 563,522 is arranged for the measurement of the total field intensity and the two rectangular components thereof by surrounding the alloy wires by solenoids through which a measured direct current may be passed. The four wires d of magnetically sensitive alloy are arranged at right angles to form a bridge network to one diagonal of which an alternating current source z is connected. The other diagonal is connected to an amplifier and indicating instrument u. Solenoids f<1>, f<2>, f<3>, f<4> surround the wires d and opposite pairs are connected in series so as to produce magnetic fields in the same direction. Energising current for the solenoids f<1>, f<4> is supplied by a battery n and can be varied by an adjustable resistance o and a reversing switchp. Measurement is effected by apotentiometer comprising a standard cell g. resistances r, r<1>, r<2> and a galvanometer s. The solenoids f<1>,f<4>arewound on tubes enclosing the alloy wires d and are mounted in grooves in an insulating disc arranged to be adjustable about vertical and horizontal axes, one parallel pair being mounted on one side of the disc and the other pair on the other side. Each alloy wire d is supported in a straight groove in which it is disposed without constraint. The disc is enclosed in a polished non-magnetic metal case with a lining of cork, baize, felt or other thermal insulator. To measure the total field intensity the disc is set in a vertical plane parallel to the magnetic meridian and the bridge is balanced by turning about the horizontal axis. A measured current is then passed through the solenoids f<1>, f<4>, and the disc is rotated to restore balance. The total field intensity may then be deduced from the amount of this adjustment and the value of the current. To compensate for mechanical imperfections the switch p is reversed and the amount of rotation of the disc in the opposite direction for balance is observed. The mean of the readings in opposite directions is used. To measure the vertical component the disc is set in a vertical plane perpendicular to the magnetic meridian with the pivotal axis vertical and balanced. The solenoids are energised and the balance is restored by rotation in the meridian plane, the switch p is reversed and balance again restored, the mean of the angular adjustment being taken and used to calculate the vertical component. For measuring the horizontal component the apparatus is similarly set up, restoration of balance being effected by rotation in azimuth.