Electrical Units

I

Introduction

Electrical Units, units used to express quantitative measurements of all types of electrostatic and electromagnetic phenomena, of the characteristics of electrical components and circuits, and in the metering of electricity supply.

II

SI Electrical Units

The electrical units now in common use are defined in the International System of Units (SI); they are the ampere (unit of electric current and an SI base unit), the coulomb (unit of quantity of electricity), the volt (unit of potential difference and electromotive force), the ohm (unit of electrical resistance), the farad (unit of capacitance), the henry (unit of electrical inductance), and the weber (unit of magnetic flux).

Historically, the ampere and the ohm were the two independent electrical units, defined in terms of the metre, kilogram, and second, which established SI electrical measurements. However, in practice the ampere is difficult to realize with sufficient accuracy, so the watt is realized instead. The watt realized electrically is compared by beam-balance experiments with the watt realized mechanically; these experiments employ a coil in a magnetic field and are devised in such a way that it is not necessary to know either the dimensions of the coil or the strength of the magnetic field. The ohm is realized from the metre via the Thompson-Lampard calculable capacitor, whose value can be changed by an amount that depends only on the linear displacement of a component. The ampere and volt can be deduced from combinations of the watt and ohm.

In all the practical electrical units the conventional prefixes of the metric system are used to indicate fractions and multiples of the basic units. Thus a picofarad is a trillionth (10^-12) of a farad, a microampere is a millionth of an ampere, a millivolt is a thousandth of a volt, and a megohm is 1 million ohms.

III

The Volt and the Ohm

The determination of electrical units in terms of SI units is very costly and laborious, and is only undertaken at intervals of several years by national standards laboratories. Therefore the volt and ohm are now routinely derived from fundamental quantum effects that are significantly more reproducible and stable than direct determinations from SI units.

The volt is derived from the AC Josephson effect in which the potential difference between two superconductors separated by a narrow gap assumes discrete values related only to the frequency of electromagnetic waves irradiating the junction and to a precisely known quantity, the Josephson constant (value 483,597.9 GHz/V).

The ohm is derived from the quantum Hall effect: certain semiconductors at low temperatures and in high magnetic fields have a resistance that is quantized in steps related to another known quantity, the von Klitzing constant (value 25,812.807 W). The SI values of the Josephson and von Klitzing constants are the 1990 consensus values determined from absolute measurements.

Voltage standards, such as standard electric cells, are calibrated in terms of the volt derived from the Josephson effect, and standard resistors are calibrated against the ohm derived from the quantum Hall effect. These standards are then used by industry to calibrate the scales of instruments used to measure electrical quantities.

See also Electric Meters; Measurement.