Automatic magnetic field measuring apparatus using NMR principles

An apparatus for measuring an external magnetic field, comprising:

• an ancillary magnetic field measuring means having a predetermined accuracy capability for providing a first magnetic field estimate;
• bias range determination means which uses said first magnetic field estimate and said accuracy capability to define a range of bias values;
• bias application means, responsive to said range of bias values, for superimposing a corresponding range of bias magnetic fields onto said external magnetic field, so as to cause nuclear magnetic resonance in a substance disposed in both said external magnetic field and a resonance bias magnetic field contained within said range of bias magnetic fields, said resonance bias magnetic field caused by a resonance bias value obtained in said range of bias values;
• and
• magnetic field computing means for computing the magnitude of said external magnetic field based on which bias value in said range of bias values is said resonance bias value which causes said nuclear magnetic resonance.

A method for measuring an external magnetic field, comprising the steps of:

• disposing in said external magnetic field a substance capable of displaying nuclear magnetic resonance;
• surrounding said substance with a high-frequency coil;
• measuring said external magnetic field to a certain degree of accuracy with an ancillary magnetic field measuring technique to produce a first estimate of the magnitude of said external magnetic field;
• determining from said first estimate of said external magnetic field an estimated magnetic resonance frequency;
• applying a high-frequency voltage of said estimated magnetic resonance frequency to said high-frequency coil;
• determining from said first estimate of the magnitude of said external magnetic field a range of values of bias coil currents to apply to bias coils surrounding said substance to cause nuclear magnetic resonance in said substance;
• applying said bias coil currents;
• measuring resultant variation in inductance of said high-frequency coil;
• determining the bias coil current for which the variation of inductance of said high-frequency coil is a maximum;
• computing the magnitude of said external magnetic field with at least as much accuracy as said first estimate of said magnitude, using the frequency of the high-frequency voltage and using the values of the one of said bias currents for which the variation of inductance of said high-frequency coil is a maximum;
• and
• displaying said magnitude of said external magnetic field.