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mho

A unit of electrical conductance. Symbol, ℧, and sometimes Ω⁻¹. Conductance in mho being the reciprocal of resistance in ohms, mho is ohm spelled backwards! The mho became obsolete when the 14ᵗʰ CGPM formally added the siemens to SI in 1971, as the units are identical.

The term “gemmho” was sometimes used for what is now called a microsiemens.¹

According to the current national standard in the United States², the mho is not to be used. The siemens should be used instead.

1. M. L. McGlashan.
Physico-Chemical Quantities and Units.
London: The Royal Institute of Chemistry, 1968.

Page 20.

2. IEEE/ASTM SI 10™-2002.
American National Standard for Use of the International System of Units (SI): The Modern Metric System.
New York: IEEE, 30 December 2002.

See Section 3.3.3.