The need to measure the strength of magnetic fields in science and industry has resulted in the production of magnetometers of many types. Early magnetometers consisted of spring-loaded magnets in which the field strength was measured by the extension of the spring. They were superseded by electronic systems, the most common being the proton, fluxgate, and Hall effect magnetometers.
Proton magnetometer
The proton magnetometer is probably the most accurate type of magnetometer available, and it relies for its operation on the alignment of protons—the positively charged particles in atoms—within a proton-rich source such as kerosene, which has a comparatively large number of hydrogen atoms. In one form of this instrument, suitable for measuring weak magnetic fields, the kerosene, which is contained within a bottle, is magnetized to a high level by means of surrounding coils. When it is fully magnetized, the protons are all aligned in the same direction. The moment the exciting field is removed, the protons begin to fall back to their original random orientation, and as they do so, they induce an alternating voltage in a sensing-coil system. The frequency of this voltage is a measurement of the surrounding magnetic field.
An alternative form suitable for use in strong fields employs a small sample of water or heavy water (D2O). The sample is exposed to a radio frequency (RF) field (1 to 1,000 MHz). At one specific frequency, energy is absorbed from the radio frequency (RF) field. This frequency is again a measure of the field strength.
The main advantage of the proton magnetometer is that the frequency can be measured very accurately, and therefore a very precise field strength can be obtained. However, it cannot be used to measure a rapidly varying or alternating field, the field direction, or, because of the recycling time, the field at any instant in time.
Fluxgate magnetometer
The fluxgate magnetometer depends for its operation on the rapid alternating current (AC) magnetization of a pair of high permeability (easily magnetizable) cores. Each core has a primary and a secondary winding, one outside the other. An AC current applied to the primary magnetizes the core, which in turn induces a current in the secondary windings. If an external field is present, the core will be magnetized more, and by using two cores arranged so that their outputs will reinforce each other, the signal is doubled for a given external field, showing up as an AC voltage of twice the original frequency.
Magnetometers based on this principle can measure fields over a small area, as small as 0.20 × 0.08 in. (5 × 2 mm), can detect rapidly varying fields, and can measure field direction as well. They are ideally suited to the detection of sunken objects such as ships, bombs, and mines. Their directional properties make them useful aboard aircraft as compasses, with one on each wingtip. Other uses include measuring magnetic fields in deep space and detecting weak magnetic fields in geological and archaeological specimens.
