Measurement

Next we must consider how magnetic field is measured. Field-measuring instruments are known as ‘gaussmeters’ (10⁴ gauss =1 tesla). The magnitude and direction of a uniform, steady field can be determined absolutely using a search coil or a rotating-coil fluxmeter (Fig. 1). The principle is that a transient or alternating emf ε is induced according to Faraday’s law, which follows from

ε = −NdΦ/dt, .........(1)

where N is the number of turns on the coil of area A. The flux density in air B₀ = Φ/A is deduced by integrating the emf, B₀ = (1/NA) ∫ Edt as the search coil is removed from the uniform field to a region where the field is zero. When measuring a pulsed field, there is no need to move the coil. Likewise, a coil rotating about an axis perpendicular to B₀ generates an alternating emf ε =−NAωB₀sinωt. These measurements are absolute, but inconvenient.

Figure 1: Some methods of measuring magnetic fields: (a) search coil, (b) rotating coil fluxmeter, (c) Hall probe and (d) nuclear magnetic resonance probe.

In practice, a Hall effect or magnetoresistance sensor is generally used to measure magnetic field. The active area of the semiconducting Hall probe is of order 1 mm², and it generates a voltage V_H = B₀I/n_cet, where I is the sensing current, t is the thickness of the semiconductor and n_c is the carrier density. The probe needs to be calibrated, and possibly corrected for temperature fluctuations, but the Hall voltage is linear in magnetic field. Accuracy is about 1%. Magnetoresistance sensors, can be based, for example, on spin valves with crossed anisotropy. Like fluxgate sensors, which depend on the asymmetric saturation of soft magnetic wires or thin-films elements, they are good for fields below 100 A m⁻¹ but they have the advantage of a high bandwidth.
Much greater accuracy and precision is possible by measuring the NMR frequency of rubidium vapour, for example, which is ν =13.93 MHz T⁻¹ for the ⁷⁷Rb nucleus, or even water, where the resonance frequency is 42.58 MHz T⁻¹ for the H nucleus, better known as the proton.

مواضيع ذات صلة

عجائب المجالات المغناطيسية

المجال المغناطيسي الارضي

Planetary magnetism

Therapy and treatment

Cellular biology

Magnetotaxis

Magnetic field effects

Electrodeposition

Magnetic memory

Materials for spin electronics

Actuators

Sensors