Patent GB-631535-A (GB)
Title: Improvements in or relating to magnetometers
IPC: G01V3/08
Filed: 1947-08-01
Abstract: 631,535. Magnetic measurements. WESTERN ELECTRIC CO., Inc. Aug. 1, 1947, Nos. 20985 and 20986. Convention dates, Sept. 25, 1945. [Class 37] [Also in Group XL (c)] In apparatus for measuring and/or indicating the strength of magnetic fields, a detector magnetometer of the kind comprising a long wound core of low retentivity and high permeability is automatically oriented along the direction of the field by motors controlled by signals derived from two similar orienting magnetometers mounted perpendicularly to each other and to the detector magnetometer. A signal derived from the latter, proportional to the field strength, operates indicating or recording means, the residual signals from the orienting magnetometers being squared, rectified and injected into the circuit of the detector magnetometer or the indicator to compensate for small deviations of detector alignment, from the field direction. In Fig. 1, a detector magnetometer and two orientating magnetometers MD, MT, MA are gimbal mounted with their magnetic axes mutually perpendicular (Fig. 5, not shown), so that the axis of the detector magnetometer may be aligned along the direction of maximum field strength. The magnetometer windings are energized through slip-rings from an alternating current source 2 of frequency F through a harmonic filter 3 and inductances LA, LT, LD across which second harmonic voltages are developed proportionally to the field strength components along the respective magnetometer axes. The harmonic voltages due to magnetometers LA, LT pass through filters 4 and amplifiers 5 and 7 to modulators 8 supplied with a steady signal of frequency 2F+f from an oscillator 9, the respective outputs at frequency f passing through amplifiers 10 to energize one winding of each of the two-phase motors of the orienting mechanism 37. The other motor windings are energized in parallel through an amplifier 10 from a third modulator 8 supplied with steady signals of frequency 2F+f from oscillator 9 and frequency 2F from filter 3 through frequency doubler 11 and filter network 12. Deviations of the orienting magnetometers from positions perpendicular to the magnetic field thus develop signals of frequency f to energize the motors, which rotate the gimballed magnetometer mounting about the axes of the orienting magnetometers through flexible cables until the detector magnetometer is aligned with the field direction when no signal is developed by the orienting magnetometers. The output of the. detector magnetometer is passed through filter 4, to an amplifier 27, linear detector 28 and D.C. amplifier 30 (Fig. 6, not shown), the output of which operates a recorder 31 and visual indicator 32 through a sensitivity control 33. Small deviations of the detector magnetometer from the field direction are compensated by applying the residual signals from the orienting magnetometer amplifiers 5 to respective double triode squaring rectifiers 13, Fig. 4, each comprising a rectifier section 40B and a cathode temperature compensating section 40A biassing the rectifier. Rectified currents from the potentiometer sliders CA, CT are combined and applied to the detector magnetometer winding through a compensation adjusting variable resistance 25, in such sense as to correct errors due to misalignment of the detector magnetometer. A portion of the field to be measured may be balanced out by passing a steady D.C. through the detector magnetometer winding from network 19 comprising fine and coarse adjusting potentiometers 21, 22, the latter being ganged with variable resistance 25. The linear detector 28 introduces an offset voltage determining the net magnetometer field strength corresponding to zero of the indicator, and networks may be inserted in the amplifier 30 to make the apparatus responsive only to magnetic field changes. Compensating current may be injected into the output of detector 28 at point 18 and resistance 25 is eliminated and network 19 only is connected to the detector magnetometer. For total field measurement only, resistance 25 and network 19 may be eliminated (Figs. 2, 3, not shown). The apparatus is applicable to the detection of bodies distorting the earth's field and may be mounted in aircraft. Specification 630,750 is referred to.