Patent GB-1505182-A (GB)
Title: Device for the measurement of the horizontal components of the magnetic field of the earth
IPC: G01R33/038
Filed: 1975-04-04
Abstract: 1505182 Magnetic field measurements FRANCE ARMED FORCES MINISTER OF 4 April 1975 [6 May 1974] 13982/75 Heading G1N Two servo-magnet variometers, i.e. magnetometers of which the respective bar magnets 3, 12 are oriented in accordance with a directional magnetic field and which magnetometers measure those horizontal components of the earth's magnetic field disposed orthogonally of the directional field i.e. orthogonally of the magnet axes and along fiducial lines M1, M2 respectively, are disposed at a fixed angle relative to one another, preferably a right angle. Each variometer has Helmholtz coils B11, B21, B31, B41 and B12, B22, B32, B42 respectively providing a feedback compensation magnetic field maintaining the magnet in its aligned disposition, a field compensating for the component of the earth's field perpendicular to the directional field, a field compensating for the component of the earth's field parallel to the directional field, and the directional field. Alternatively the function of coils B11, B21 may be performed by a single coil. The variometer 1 comprises a mirror 5 fast with the magnet 3 and illuminated by a light source 6 through a lens 7 to reflect light to a system of photo-electric cells 8, the output current from which is proportional to the angle of rotation of the magnet from its alignment with the directional field. Variometer 2 has similar components 14, 15, 16, 17. The variometers are suspended by respective torsionless filaments 4, 13. A further mirror, light source and photo-electric cell system 21, 22, 23, 24 and 25, 26, 27, 28 services for setting up the variometers as described below. The device consisting of the two variometers is lowered to the sea bed with an unknown orientation relative to magnetic north, with the magnets clamped in dispositions aligned with their respective directional magnetic fields and with contacts 65À1 and 81À1, Fig. 3, open so no feedback compensation magnetic field is supplied initially from coils B 11, B12. Now the magnets are freed and rotate in response to the earth's magnetic field compounded with the directional fields set up by current from current supply box 37 flowing in coils B41, B42. The pairs of photo-electric cells 23, 24 and 27, 28 act with respective amplifiers 42, 44 and detectors 45, 46 to provide for each variometer a logic 1 or 0 depending on the sense of magnet rotation. A compensation control logic circuit 66 now responds to the logic signals from detector 45 to establish the appropriate sense amd amplitude of current supplied from box 29 to coils B31 and B22 so as to compensate for the component of the earth's magnetic field parallel to the directional field in variometer 1 and perpendicular to the directional field in variometer 2 since the circuit from box 29 is via lead 30, coils B31, lead 31, coils B22 and lead 32 back to the supply box. Variations in current amplitude are effected sequentially under control of clockpulses every ten seconds from clock 76 via monostable 75, AND gates 62, 63, flip-flops 68, counter 73, OR gates 69 and relays 70 to selectively switch resistors 71À1, 71À2 and so on into circuit between a potential source and the coils until the two cells 23, 24 are equally illuminated. This indicates that the magnet 2 is aligned with the directional field and NAND gate 61 supplies no further output signal to the AND gates 62, 63. Store 77 records the binary word representing the resistors 71 which are connected in circuit with the coils and this binary word represents one component H 1 of the compensatory field. Similarly a compensation control logic circuit 79 responds to the logic signals from detector 46 to establish the appropriate sense and amplitude of current supplied from box 33 to coils B21 and B32 so as to compensate for the component of the earth's magnetic field perpendicular to the directional field in variometer 1 and parallel to the directional field in variometer 2 since the circuit from supply box 33 is via lead 34, coils B21, lead 35, coils B32, lead 36 and back to the box 33. A corresponding store in circuit 79 to that 77 stores the binary word representing the second component H 2  of the compensatory field. If these components are H 1 , H 2  then since they are orthogonal the intensity Ho of the earth's field is given by H o =(H 1 <SP>2</SP> + H 2 <SP>2</SP>)<SP>¢</SP> and the cosine of the angle a, between the magnet 3 and magnetic north is H 1 /H o . Once the correct compensating fields have been established the generator 83 divides by two the current from box 37 to retain a reduced intensity of directional field thus increasing the sensitivity of the variometers. Then the logic zero from NAND gate 61 acts via inverter 64 and relay 65 to close contact 65.1 to supply current to coils B 11 to establish automatic counterreaction compensation for transient variations of the earth's field. Similarly for contact 81.1 and coil B12.