General

Measurement of a quantity consists either of its comparison with a unit quantity of the same kind or of its determination as a function of quantities of different kinds whose units are related to it by known physical laws. An example of the first kind of measurement is the evaluation of a resistance (in ohms) with a Wheatstone bridge in terms of a calibrated resistance and a ratio. An example of the second kind is the calibration of the scale of a wattmeter (in watts) as the product of current (in amperes) in its field coils and the potential difference (in volts) impressed on its potential circuit.

The units used in electrical measurements are related to the metric system of mechanical units in such a way that the electrical units of power and energy are identical with the corresponding mechanical units. In 1960 the name Syst me International (abbreviated SI), now in use throughout the world, was assigned to the system based on the meter-kilogram-second-ampere (abbreviated mksa). The mksa units are identical in value with the practical units-volt, ampere, ohm, coulomb, farad, henry-used by engineers. Certain prefixes have been adopted internationally to indicate decimal multiples and fractions of the basic units.

A reference standard is a concrete representation of a unit or of some fraction or multiple of it having an assigned value which serves as a measurement base. Its assignment should be traceable through a chain of measurements to the...