Instruments detail

Number of axes of detection. There are no sensors that directly assess a resultant field in a random direction in space; sensors generally measure the field in one direction. A meter may have one sensor. If this is aligned by the user with the direction of maximum field it will give a reading of the maximum field in a single direction; the overall resultant field may be between 1.0 and 1.41 times this value depending on the degree of polarisation. If the meter has three orthogonal sensors, the resultant field can be obtained from the three values measured by root-sum-of-squares addition: Resultant = . This resultant value is independent of the orientation of the meter, vastly simplifying use of the meter.
More on elliptically polarised fields

Measure of field. Various measures of a sine wave are possible, eg peak, rectified average, root-mean-square (rms). For a single frequency, ie a pure sine wave, these can be scaled to give the same result, but in the presence of harmonics they can differ considerably. In the absence of a known biophysical mechanism, there is no conclusive basis for asserting that any one measure is correct. However, by analogy with other areas of measurement science, there is an assumption that rms is the preferred measure. Some meters capture the actual waveform for future analysis.

Frequency response. Instruments may be sensitive to a single frequency eg 50 Hz or 60 Hz or to a range of frequencies. If sensitive to a range of frequencies, the response may be flat or may be proportional to frequency. A flat frequency response between 20 or 30 Hz and a few kiloherz is generally regarded as suitable for many general purpose measurements.

Size of sensors. Sensors can be made small – a few millimetres - and therefore capable of investigating variations of field over small distances. However, there may also be times when it is desirable to use larger sensors which measure the average field over their area.

Readout and logging. Meters may have analogue or digital displays. They may only display a value in real time, or they may be capable of logging values with various degrees of sophistication, and of calculating various parameters of the field such as averages or maxima.

Given the variations in facilities provided by a meter, there is an inevitable variation in size, weight, and battery consumption. Some meters are most suitable for detailed surveys by experts; others are small and light enough to be worn by volunteers for extended periods.

There is no “correct” or “best” meter. The best meter to use depends on the purpose it is to be used for.

Measuring magnetic fields
There are three different sensors widely used for measuring magnetic fields:

Search coils. The simplest meters measure the voltage induced in a coil of wire. For a sinusoidally varying magnetic field, B, of frequency f, the voltage, V, induced in the coil is given by:

where f is the frequency of the field and A is the area of the loop, and is the component of B perpendicular to the loop.

The voltage induced by a given field increases with the addition of more turns of wire or of a ferromagnetic core. To prevent interference from electric fields, the magnetic field probe must be shielded. If the meter is used for surveys or personal exposure measurements, frequencies lower than approximately 30 Hz must be filtered out to remove voltages induced in the probe by the motion of the meter in the earth’s magnetic field.

Fluxgate magnetometers. These detect a magnetic field by the asymmetry it produces in a ferromagnetic material deliberately driven in magnetic saturation alternately in opposite directions at a high frequency.

Hall-effect devices. The sensor is designed to measure the transverse Hall voltage across a thin strip of semiconducting material carrying a longitudinal current.

Most practical instruments for power frequencies use search coils, either a single coil or three orthogonal coils. The coils themselves can either be made as small as possible, with a ferromagnetic core to increase sensitivity, for use in personal exposure meters where size and weight are important criteria; or they can be larger, often 0.1 m across, to increase sensitivity and provide some spatial averaging. Fluxgate magnetometers cannot be made as small or as cheap, but have the advantage of responding to dc fields as well as ac. Hall devices are little used as their resolution is poorer and they suffer from drift but have uses at higher fields.