Patent GB-727129-A (GB)
Title: Measuring apparatus including means producing magnetic bias fields and alternating electromagnetic fields
IPC: G01R33/24
Filed: 1953-12-03
Abstract: 727,129. Magnetic tests. GENERAL ELECTRIC CO. Dec. 3, 1953 [Dec. 6, 1952], No. 33666/53. Class 37. Apparatus for measuring the strength of a magnetic field by utilization of the fact that the relationship between the Lamor frequency of precession v of the nuclei in the presence of a magnetic field H is given by 2#v=constant x H, comprises means for applying a radio frequency magnetic field to a sample of material containing hydrogen atoms, e.g. water, in a direction perpendicular to that of an audio frequency modulated constant field, so that when the radio frequency is the same as the Lamor frequency of the hydrogen nuclei the impedance of a coil associated with the water sample varies, the frequency at which this occurs being indicative of the strength of the constant magnetic field. As shown (Fig. 1) four identical permanent magnets 1, 2, 3, 4 are located in axial alignment with equal gaps between 1, 2 and between 3, 4, the fields in the gaps being in opposite directions, and modulated by coils 11, 12 fed from an A.F. oscillator 13. Coils 7, 8 wound on sealed glass tubes 5, 6 respectively containing water are located in the gaps with the coil axes perpendicular to the direction of the fields, and the coils are energized from an R.F. oscillator 15 in bridge circuits 14, 20 respectively. The outputs of the bridges are demodulated at 17, 21, amplified at 18, 22 and rectified at 19, 23 to give direct currents which are fed to coils 9, 10 respectively wound round magnets 1, 4, to maintain the field strengths through sample tubes 5, 6 at that for magnetic resonance of the nuclei of the hydrogen atoms in the water. In the presence of an external field having a component along the axis of magnets 1-4, the direct currents fed to coils 9, 10 are unequal, and their difference, shown on a meter 24, is indicative of the strength of the additional field component. In a modification (Fig. 2) the A.F. modulated magnetic field causes the water sample in tube 32 to pass through the magnetic resonance point, and the change in impedance of associated coil 34 varies the output of R.F. oscillator 36 to provide an A.F. signal which, after amplification at 38 and rectification at 39 is fed to a reactance tube 37 to keep the frequency of oscillator 36 at the magnetic resonance value. The frequency of a similar R.F. oscillator 40 is similarly controlled, so that in the presence of an additional field the frequencies of oscillators 36, 40 are different, and after amplification are mixed at 44 to give a difference signal at 46 indicative of the field strength. In an embodiment for determining the field gradient of an auxiliary field (Fig. 3) the A.F. modulated bias fields between magnets 47, 48 and between 49, 50 produce magnetic resonance in similar samples 54, 55 in the presence of R.F. fields supplied by oscillators 58, 64 respectively whose frequencies are maintained at magnetic resonance by reversible motors 62, 68 driving the tuning condensers 59, 65. In the absence of the external field the R.F. frequencies are identical, but with the application of the auxiliary field the R.F. frequencies are different and the difference is indicated on a frequency meter 72, or by a mechanical differential 73 indicating the difference between the rotations of motors 62, 68. In a modification the motors rotate auxiliary bar magnets located near the sample tubes to equalize the field strengths, the difference between the rotations of the motors being mechanically indicated.