Patent GB-904260-A (GB)
Title: Method and apparatus for measuring magnetic fields
IPC: H01S1/00
Filed: 1959-10-06
Abstract: 904,260. Radiation magnetometers; modulating light. FRANKEN, D. A., and TEXAS INSTRUMENTS Inc. Oct. 6, 1959 [Oct. 7, 1958], No. 33835/59. Class 40 (3). [Also in Groups XXXVI and XL (a)] In a method of measuring a unidirectional magnetic field 28, helium contained in a vessel 32 situated within the field to be measured is continuously converted to the metastable <SP>3</SP>S 1  state, for example, by passing an electric discharge through the vessel 32, and is operated on by resonance radiation from a helium discharge tube 35 to " align " the metastable <SP>3</SP>S 1  atoms, i.e. to bring them to a state in which there is a maximum transmission of resonance radiation through the gas in the vessel 32, this gas also being subjected to a variable radio-frequency magnetic field which at a frequency dependent upon the value of the magnetic field being measured tends to destroy the " alignment " whereby transmission of the resonance radiation from the tube 35 through the vessel 32 is reduced, this reduction being detected by means of a photo-electric detector 16. The radio-frequency magnetic field is produced by a coil 30 surrounding the vessel 32 and energized from a source 24. In the arrangement illustrated a low-frequency alternating magnetic field produced by a pair of Helmholtz coils 22 is superimposed on the field 28. This has the result, within certain values of the radio frequency from source 24, of sweeping the magnitude of the unidirectional magnetic field in which the vessel 32 is situated periodically through the value necessary to achieve a reduction in resonance radiation transmission; such reduction can conveniently be displayed in a circuit 20 which may comprise a cathode-ray tube having a time-base synchronized with the alternating magnetic field. Alternatively a similar result may be achieved by the superposition of a frequency modulation on the variable radio-frequency power supply 24. As shown, a directcurrent power supply is supplied to the discharge vessel 32; alternatively high-frequency power, X-rays or radiation from radioactive material placed in or near the vessel 32 may be used for " aligning " the metastable <SP>3</SP>S 1  atoms. An alternative construction, Fig. 5, comprises a central discharge tube 52 producing resonance radiation and surrounded by a metal screen which is continuous with the outer conductor of a coaxial cable supplying the discharge tube. Longitudinal slots over the central portion of the screen allow resonance radiation from a central portion of reduced cross-section of tube 52 to pass to an annular discharge vessel 66 containing helium to " align " the metastable <SP>3</SP>S 1  helium atoms produced by a discharge between electrodes 46. A coil 50 energized from a radiofrequency source surrounds the vessel 66 and a detector comprising a plurality of lead-sulphide plates 48 surrounds that. Preferably a radiofrequency source is employed for producing the discharge between electrodes 46. Sheets of polarizing material may be interposed between the two tubes 52 and 66 and around the tube 66 to discriminate against scattered radiation produced within the tube 66.