Patent GB-1390924-A (GB)
Title: Magnetic sensing apparatus and methods
IPC: G01R33/24
Filed: 1971-08-24
Abstract: 1390924 NMR magnetometers UNITED KINGDOM ATOMIC ENERGY AUTHORITY 23 Aug 1972 [24 Aug 1971] 39710/71 Heading G1N In a magnetometer utilizing nuclear magnetic resonance a strong magnetic field is applied to a specimen to polarize its nuclear magnetic moments, then the field is removed and the moments precess about any residual magnetic field, and finally a measuring field is applied and precession of the moments about that field is detected by a tuned coil. The measuring field is of a lesser magnitude than the polarizing field, and must be homogenous; the polarizing field is applied for a period which is a few times the specimen relaxation time, and is terminated rapidly. The measuring field should have a fast rise time. Measuring fields:-In Fig.3 the sequence of operations for field measurement are controlled by pulse generator 33. At the commencement of the measuring a high voltage pulse is applied to coil 12 by triggering SCR32 so as to initiate the polarizing field. This field is sustained by power supply 20, and is made homogenous by coils 40 connected to coil 12. When the power supply is switched off by pulse generator 33 the polarizing field decays, and the moments of the specimen, which may be water with MnSO 4 , precess around the field to be measured. The measuring field is then initiated by the pulse generator which activates supply 20 and SCR32 to give a fast rise time followed by a constant current. The r.f. signal from specimen 4 is picked up by a tuned coil wound round it and heterodyned at receiver 8 before display on oscilloscope 54, while an integrator 56 measures the signal. Homogeneity of the measuring field can be enhanced by reversing its direction during its initiation. Separate coils may be used to generate the  polarizing and measuring fields, but must be accurately aligned; the alignment can be eased if the polarizing field is applied by both coils. Magnetic screening, e.g. by Á metal or a superconductor may be used to eliminate the earth's field. The magnitude of the field to be measured can be found by using nulling coils; its direction can be found by turning the polarizing and measuring coils until their fields are aligned with the field being investigated, whereon no precession occurs. The apparatus can thus be used as a compass, and may have a servo-system for moving the coils until precession ceases. During field measurements, the moments will only precess around the measuring field if they were not aligned with it when the field was initiated. This alignment depends on the extent to which the moments have precessed around the field to be measured. If the magnitude of that field is roughly known, the measuring field may be applied at a time calculated to cause precession. Otherwise if no precession is detected it is necessary to repeat the measurement with a  different delay between the polarizing and measuring fields.  Measuring rotation:-If the apparatus is screened from all external fields there will be no detected precession unless rotation occurs during the time between the end of the polarizing field and the start of the measuring field. Rotation can thus be measured, and a magnetic gyroscope may be formed.