mho

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[Etymology: reverse spelling of ohm] electric conductance (Metric) The conductance of a circuit in mhos is the reciprocal of its resistance in ohms, i.e. it equals the ratio of the current produced in amperes between two points of a conductor to the r.m.s. potential difference in volts across these points (the conductor not being the seat of any electromotive force).

Metric-m.k.s.A. 1948 Ω^-1 (= m^-2·kg^-1·s³·A² in base terms). The mho is identically the SI unit siemens in non-inductive circumstances; the name mho has not been retained within the SI, although it is coherent with the contemporary system.

Metric-c.g.s. See abmho; statmho. See also practical unit.

History

The mho entered use in 1883, joining the already established practical units derived from the e.m.u. system. This was the practical mho, = 10^-9 abmho. Like all the e.m. units, the abmho was itself defined ultimately in terms of purely mechanical units.

Because the explicit laboratory specifications established for the ampere, ohm, and volt were subsequently found to be slightly discrepant from the intended, so was the mho, as a unit derived from them. The International Electrical Conference of 1908 covered the discrepancy by adopting the distinct name international mho. Because of experimental vagaries, the value for conversions is normally referred to as the mean international mho, = 0.999 510~ S. There is also the US international mho, = 0.999 505~ S.

At the IEC in 1933, the distinct name siemens was adopted in place of the contrived name mho, within the m.k.s. system.

With the implementation of the Metric-m.k.s.A. system in 1948, and its basing of electrical units on an ampere compatible with the original absolute units, the modern unit became essentially the old practical mho.

IN BRIEF: n. - A unit of conductance equal to the reciprocal of an ohm.

Tutor's tip: Note: A "mho" is (an electrical unit); "mot" is (a witty remark), and "mow" is (to cut down grass).

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a practical unit of electric conductance equal to the reciprocal of the impedance measured in ohms. For circuits containing only noninductive resistance the siemens is the preferred unit.

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