Patent GB-1246792-A (GB)
Title: Improvements in and relating to apparatus for measuring magnetic fields by gyromagnetic resonance
IPC: G01R33/24
Filed: 1963-12-03
Abstract: 1,246,792. Gyromagnetic resonance magnetometers. SUB-AVIATION SOC. NATIONALE DE CONSTRUCTIONS AERONAUTIQUES. Dec. 3, 1963 [Dec. 11, 1962], No. 47809/63. Heading G1N. In apparatus for measuring magnetic fields by gyromagnetic resonsance of a nucleus or electrons produced by an HF current or monochromatic light, the apparatus is additionally provided with two coils having a zero mutual induction coefficient, one coil, denoted as the excitation coil, bringing the oscillations of the various protons or electrons into phase and being connected to the output end of an amplifier, the other coil, denoted as the detection coil being connected to the input end of the amplifier. Thus the frequency of the excitation coil is a function of the magnetic field to be measured, such an arrangement being disclosed in Specification 907, 013. Maximum sensitivity is obtained when the magnetic field to be measured is normal to the axes of the excitation coil and the detection coil and measurement is impossible when the magnetic field deviates at more than 50 degrees from the normal. The present invention minimises this drawback by rotating the axis of the excitation coil in a plane normal to the axis of the detection coil and controlling this rotation so that it brought into a direction at right angles to that component of the magnetic field to be measured which lies in the said normal plane. Fig. 1 shows a nuclear resonance magnetometer having a container 1 enclosing a gyromagnetic substance, an excitation coil 2 having a field axis XX and a detection coil 3 having a field axis YY. The normal to XX and YY is denoted by ZZ and it is the object of the invention to automatically bring the normal ZZ into the direction of the magnetic field to be measured. In the circuit diagram, Fig. 2 crossed excitation coils 4, 4a and 5, 5a are disposed along two mutually perpendicular axes contained in a plane normal to the detection coil 3. The coils 4, 4a, 5, 5a are energized by a variometer 6 such that rotation of the variometer rotatable winding causes the excitation axes to turn in the plane of the axes of the coils whilst its stationary windings are connected to the coils. In order to servocontrol the position of the excitation axis and bring it at right angles to that component of the magnetic field to be measured which lies in the plane containing the crossed coils 4, 4a, 5, 5a, a resistor 7 and a capacitor 8 apply to the coils a low amplitude voltage of a very low frequency, e.g. 10 c/s, delivered by an oscillator 9. Thus a rotating field 10 c/s is generated in the plane of the coils, this rotating field combining vectorially with the external field and being added thereto when its direction and sign are the same. Hence there is obtained from the output of the nuclear resonance oscillator comprising the self-oscillator 10 and amplifier 11 a signal at the resonance frequency corresponding to the field to be measured modulated by 10 c/s frequency. The amplifier 11 is connected through a frequency meter 12 to a filter amplifier 13 where the 10 c/s signal is removed. The 10 c/s signal and signal from oscillator 9 are applied after rotation in a phase-shifter 14 to a phase comparison device 15. The system is initially adjusted so that when the 10 c/s field is coincident with say coils 4, 4a and the variometer 6 provides a field aligned with the axis of coils 4, 4a, the 10 c/s signal from filter amplifier 13 is in phase with the signal from the phase-shifter 14 is the field to be measured is normal to the axis of the coils 4, 4a. This requirement is met by adjusting the phaseshifter 14 so that the signal from device 15 is zero. When the apparatus is now subjected to the magnetic field to be measured of which the component lying in the plane 4, 4a, 5, 5a makes an angle a with the normal to the axis of the coils 4, 4a, the 10 c/s signal from filter amplifier 13 will be phase-shifted by a relative to the signal from the phase-shifter 14. The phase comparison device 15 provides at its terminals 16, 17 a D. C. voltage proportional to the phase-shift and of a sign dependent upon the sign of a. The D. C. voltage rotates a servomotor 18 driving both the rotatable field winding of variometer 6 and the phase-shifter 14, the motor 18 stopping when the output from device 15 is zero. The magnetic field measurements are indicated directly by a D. C. galvanometer (not shown) connected to the meter 12. The apparatus is suitable for towing behind an aircraft and may measure the earth's magnetic field or detect submerged objects.