practical units

(′prak·ti·kəl ′yü·nəts)

(electromagnetism) The units of the meter-kilogram-second-ampere system.

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electromagnetics (Metric) Any of the various units of more practical size derived, by multiplying with powers of ten, from the absolute units of the e.m.u. system. These were accorded the unqualified familiar names volt, etc., instead of the abvolt (and statvolt of the e.s.u. system).

ampere	= 10-1 abampere
coulomb	= 10-1 abcoulomb
farad	= 10-9 abfarad
henry	= 109 abhenry
mho	= 10-9 abmho
ohm	= 109 abohm
pramaxwell	= 108 maxwell, = 109 abvolt·second
volt	= 108 abvolt

which survive, at like size in SI, the mho as the siemens, the pramaxwell effectively as the weber, plus the absolete.

pragilbert	= 10-1 gilbert, = 1/40π abampere·turn
praoersted	= 10-3 oersted, = 1/4000π abampere·turn·cm-1

History

The members of the absolute system of units developed under the title electromagnetic units, forming the metric e.m.u. system, were mostly far from the amounts occurring in everyday electrical work; most were minute. This prompted the first International Electrical Conference
[Nature Vol. 24, 512 (1881)] to define, simultaneously, ‘practical units’ that were multiples, by appropriate powers of ten, of the absolute c.g.s. ones.

To make the realization of the practical units more truly practical, it was agreed to specify key ones in practical experimental terms, giving the Hg ohm expressed in terms of the resistance of a specified column of mercury, and a volt set by a Clark cell. These two definitions, plus coherent derivatives for current (ampere), for quantity of electricity (coulomb), and for capacitance (farad), were established in 1881. By 1900 these specifications were found to be at minor variance with the previous ones, and not mutually self-consistent. The consequence was the agreement in 1908 on the distinctively labelled international units, which varied slightly from the intended practical units, based on the Ag amp expressed as a depositional rate of silver.

The e.m.u. system within which the abvolt, etc., were defined was itself based on the setting of the value of magnetic permeability to 1; in 1901 it was realized that, were permeability set at 10^-7 rather than 1, the whole array of practical terms shown above fitted unchanged into the metric m.k.s. system without any multipliers. Thus, what began as part of a c.g.s. system became very readily part of the m.k.s. system, which was increasingly favoured, and ultimately the like-structured SI system.