The measurement of self- or mutual inductance. An electrical reactance such as the angular frequency (2&pgr;f, where f is the frequency) times self- or mutual inductance is the ratio of the alternating voltage having the appropriate phase, which appears across specified terminals, to the current through the device. Commercial instruments often measure inductance from this ratio by comparing it with the voltage-to-current ratio associated with a noninductive resistor. See Electrical impedance
Some practical precautions must be taken if accurate results are to be obtained. Any magnetic field associated with the inductor must not interact significantly with magnetic or conducting material in the vicinity of the inductor, since the field, and therefore the inductance, would be altered. The varying magnetic field of an inductor will induce eddy currents in any nearby conducting material, which will in turn produce a magnetic field which interacts with the inductor and measuring system. Errors in a measurement of inductance may also arise from the interaction of the magnetic field of an inductor with the rest of the measuring circuit. Capacitance to other parts or to the surroundings of an inductor arising from its associated electric field will inevitably affect the impedance or apparent inductance of an inductor by a frequency-dependent amount but capacitive currents associated with screening of the measuring circuit can be routed in such a way as not to affect the measurement. See Eddy current, Inductor
If the magnetic circuit of an inductor includes magnetic material whose permeability depends on its previous magnetic history, or the magnetic flux caused by a direct current flowing simultaneously in the coil, its inductance will also be current- or history-dependent, and these conditions must be specified if the measurement is to be meaningful.
The electrical property of self- or mutual inductance is only defined for complete circuits. Since a measuring device or network forms part of the complete circuit when it is connected to an inductor to perform a measurement, it is necessary to ensure that either the inductance associated with the measuring circuit is negligible or that the measured quantity is defined as the change in inductance occurring when the unknown is replaced by a short circuit. The former procedure is usual for mutual inductors, and the latter for self-inductors. See Electrical measurements, Inductance