Patent GB-592394-A (GB)
Title: Electromagnetic induction device
IPC: G01R33/04
Filed: 1943-07-15
Abstract: 592,394. Magnetic tests and measurements. BENDIX AVIATION CORPORATION. July 15, 1943, No. 11568. Convention date, May 29, 1942. [Class 37] An electromagnetic induction device or magnetic pick-up comprises an elongated core of magnetically permeable material adapted to be traversed by a uni-directional magnetic field and provided with an input and an output winding, the input winding being connected to an A.C. source for periodically saturating the core so that an A.C. voltage is generated in the output winding from the uni-directional magnetic field, and the core having predetermined initial minimum and maximum permeabilities and a predetermined ratio of length to crosssectional area such that the output voltage is a linear function of the strength of the unidirectional field. Such a device may be used as a magnetometer to measure the intensity of a magnetic field, or as a magnetic pick-up for remote indication or automatic steering. A pair of spaced parallel core members 11a, 11b, Fig. 2, of magnetically permeable material having substantially identical magnetic characteristics are arranged to be traversed by the uni-directional magnetic field to be measured. On each of these members are wound separate primary windings 12a, 12b, which are connected in series opposition and energized by alternating current. A secondary winding 13 is then wound about both of the core members 11a, 11b as though they were a single unit and is connected to a voltmeter 15. The fundamental and odd harmonic components in each core are equal in amplitude and opposite in phase, so that only the even harmonic voltages are induced in the secondary winding 13. The amplitude and polarity of the even harmonics are determined by the magnitude and relative direction of the constant uni-directional magnetic field being measured, as these factors determine the operating point of the magnetization curve. The windings 12a, 12b are wound so as to cover their cores 11a, 11b completely in order to saturate all the magnetic material of the cores. The initial permeability of the core is between 10,000 and 21,000, and the maximum permeability between 61,500 and 10,500. The ratio of length to an equivalent diameter of the cross-sectional area lies between 30 to 1 and 60 to 1. In an' example, each of the core members 11a, 11b consists of two laminations 4 in. long, 0.015 in. thick and 0.090 in. wide. The secondary winding 13 has 1200 turns. The magnetic field pick-up unit shown in Fig. 2 may be combined with a plurality of like units to provide a single composite unit for use as the earth inductor of a compass. For example, three such pick-up units may be arranged to form a triangle with their secondary windings connected in delta to form a three-phase output circuit. In a modification, six units extend radially outwards from a common point and are equi-angularly spaced, their secondary windings being starconnected. Three units as shown in Fig. 2 may be arranged mutually at right angles with their cores intersecting as shown in Fig. 5 so that they will not react with each other, to form a composite non-directional device for measuring the strength of a magnetic field, such as the earth's field, without orienting the device with respect to the field. With such an arrangement the field strength is proportional to the sum of the squares of the voltages induced in the secondary windings of the three units. Each secondary winding is in two parts 13a, 13b, Fig. 10, arranged on each side of the crossing point of the cores and is connected in series with a heater resistance 26, 27, 28. These heat three thermo-couples 29, 30, 31 arranged in series with each other and with an ammeter 32, which may be calibrated in terms of field strength. Specifications 549,528, 550,399, [Group XXXVIII], and 550,779 are referred to.