Patent GB-1083370-A (GB)
Title: Magnetometer
Inventor: ROBBIANO PIERINO PAUL
IPC: G01R33/24
Filed: 1965-11-12
Abstract: 1,083,370. Magnetometer. VARIAN ASSOCIATES. Nov. 12, 1965, No.48197/65. Heading G1N. The direction of a magnetic field in azimuth and elevation is determined by utilizing its effect on the vapour in an absorption cell 18, Fig. 1 containing, e.g. Rubidium vapour, irradiated with light from a Rubidium lamp 10. The application of a weak A. C. field perpendicular to the ambient field to be measured and varying at the Larmor frequency, e.g. 240 kc/s will result in an intensity modulation of the light passing through the absorption cell at a frequency equal to the Larmor frequency, which itself is proportional to the ambient field. In order to measure the direction of the ambient field, a second modulating A. C. field is applied across the cell, whose direction is known and can be changed, resulting in a variation of the total field in which the Rubidium vapour exists and therefore a variation in the Larmor frequency. The intensity modulation of the light from the absorption cell is observed by a photo-cell 22, amplified, shifted in phase by about 90‹ and presented as a current to a solenoid 28 which produces the required weak A. C. field, in order that the process of light modulation at the Larmor frequency should continue automatically. Simultaneously a sample signal from the oscillator is fed to a frequency discriminator circuit, the amplitude of whose output is dependent on the change in the ambient magnetic field due to the modulating field applied by a coil 30. Thus if the orientation of the modulating field relative to the ambient field, e.g. the earth's field, is changed, variations in the amplitude of the discriminator output will be observed, the variations being a minimum when the two fields are perpendicular. In the embodiment, the Rubidium lamp 10 emits light of 7, 800 and 7, 948Š in wavelength which is directed through a lens 12, an interference filter 14, which only passes the Rubidium D 1  line of wavelength of 7, 948Š, a circular polarizer 16 and the Rubidium vapour absorption cell 18. The modulating field H m  is applied by coils 30, coupled to a signal generator 32 providing a low frequency A. C. signal of constant amplitude and frequency of, e.g. 500 c/s. Thus the modulating field H m  both produces an amplitude modulation of the earth's field He and a frequency modulation of the output of the absorption cell, by its variation of the total magnetic field acting on the absorption cell. It can be shown that if the modulation field and earth's field are perpendicular, the modulating frequency of the output of the absorption cell will be twice the applied modulating frequency but as the angle # between the two field decreases, the modulating frequency of the cell output will be equal to the applied modulating frequency. Also it can be seen that the magnitude of deviation of the resultant field from the earth's field will decrease as # decreases. The output from the photo-cell, which has a frequency of 240 Kc/s and is frequency modulated by the 500 c/s signal, is received by a phase detector 34 where it is compared with a signal from a variable frequency oscillator 36. This oscillator 36 is locked in phase and frequency to the absorption cell output by the application of the signal from the phase detector to a voltagevariable capacitor in the r.f. tank circuit of the oscillator 36. The output from the phase detector is then applied to an amplifier 38, a narrow bandpass filter 40, which rejects all but the 500 c/s frequency signal, and finally to an oscilloscope 42 where it is compared with a 500 c/s reference signal from the generator 32. To obtain the magnetic azimuth, the lamp, cell and coils 30 are rotated about a vertical axis until the amplitude of the signal seen on the oscilloscope is a minimum when 6 is equal to 90‹. Similarly the magnetic dip angle is obtained by rotating the apparatus about a horizontal axis. As the discriminator output signal has a 180‹ phase shift as it passes through the minimum, the direction of the field can be obtained; because this occurs as # passes through 90‹. The magnetic field direction can also be determined using the field equation for the total field H,. thus the direction of the field can be determined as a function of the amplitude of the frequency deviation of the oscillator frequency. In this case the axis of the modulating coils can be arranged perpendicular to or coaxial with the coil 28. Other elements, e.g. potassium, or caesium, may be used in this apparatus in place of rubidium.