Patent GB-721666-A (GB)
Title: Improvements in or relating to apparatus for the determination of the configuration of the earth's magnetic field
Inventor: BREWER ALAN WEST
IPC: G01R33/04
Filed: 1948-10-28
Abstract: 721,666. Automatic-control systems. MINISTER OF SUPPLY, BREWER, A. W., and ELLIOTT BROS. (LONDON), Ltd. Oct. 28, 1949 [Oct. 28, 1948], No. 28054/48. Class 38 (4). [Also in Groups XXXVIII and XL (b)] Apparatus for the determination of the earth's magnetic field in magnitude and direction by aerial survey comprises means for automatically recording the position of the aircraft and the time, and means for measuring the total strength of the earth's field, and the strength of the components in the direction of a heavenly body, e.g. the sun, and in the vertical. The apparatus for determining the total earth's field consists of three fluxgates of the saturated-core type as described in Specification 721,731 rigidly mounted in a gimbal ring assembly with their axes mutually perpendicular, two of the fluxgate controlling servomechanisms to maintain the axis of the third fluxgate coincident with the direction of the field, so that its output measures the strength of the earth's field. The apparatus for measuring the vertical component consists of a fluxgate assembly secured to a freely supported pendulum. The component in the direction of the sun is determined by a fluxgate assembly attached to a telescope associated with a photo-electric controlled servo-mechanism for maintaining the axes of the telescope and the fluxgate in the required direction. The position of the plane may be determined by a radar lattice navigation system, or by a continuous photographic record. Magnetometer circuit.-A bridge 8 (Fig. 1) containing two fluxgates 10 is fed with A.C. from an oscillator 5, the output, consisting of alternate positive and negative peaks, of different heights in the presence of an ambient magnetic field, being amplified at 15, rectified at 16, and the difference in height between the voltage peaks is amplified at 17 to give an indication on a recorder 19, and also fed in series or in parallel to a negative feed-back coil 20 associated with the fluxgates 10. An alternative circuit, Fig. 2 (not shown), measures the second harmonic content of the bridge output, the magnitude being proportional to the intensity of the ambient field, and its phase indicative of the direction of the field. Automatic following telescope, Figs. 3, 5, 7.- The telescope 35 is mounted on a partition 40 in a housing 36 constituting one ring of a gimbal mounting, the light rays passing through two apertures 41, 45 to a photo-cell 42. A shutter 46 situated between the apertures contains two slits 47, 48 at 90 degrees to each other and at 45 degrees to their common diameter, so that when the shutter 46 is rotated by a constantspeed motor 50 the slits traverse the aperture in alternatively horizontal and vertical directions. Two cams 51, 52, attached to the rotating shutter 46, are associated with switches Sp, Sa and Se so that the pulse received by the photo-cell 42 for each slit is cut in half and fed through comparison units 66, 67 to servomotors Ma, Me to correct the position of the telescope to make the half-pulses equal. A fluxgate 10a is rigidly fixed to the telescope 35 so that their axes are parallel.