A NEW METHOD OF MEASURING THE EARTH'S MAGNETIC FIELD*

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Abstract
This paper describes the measurement of the Earth's total magnetic field by a determination of the frequency of free precession around this field of the protons in a sample of water. The angular frequency of precession, ω, is related to the field F by the relationship ω=γpF where γp is the gyromagnetic ratio of the proton.A bottle of water is magnetised in a direction approximately at right angles to the earth's field, by passing a current through a coil wound around the bottle. The current is switched off rapidly, leaving the magnetic moment vector to precess around the earth's field direction, inducing in the coil a small alternating voltage. This voltage is amplified and its frequency measured by means of a counter. The accuracy of measurement of the total field is estimated at ± 0.5γ.In addition to the total field value, the form of the signal voltage contains information on the field gradient across the sample.Simple theory, experimental details and results are given.

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