MHO

Mho is a historical unit of electrical conductance equal to the reciprocal of resistance. It is defined as the reciprocal of the ohm, so one mho represents one siemens (G = 1/R, with R in ohms and G in siemens). The name mho comes from Ohm spelled backward, and the symbol most often associated with it is the inverted uppercase omega (℧). In modern practice, the siemens (symbol S) is the official SI unit for conductance.

Relation to other units: Since conductance is the reciprocal of resistance, 1 ohm corresponds to 1 siemens. The mho and the siemens are dimensionally identical; 1 mho equals 1 siemens. The primary difference is historical and conventional: siemens is the standard SI unit, while mho is a non-SI unit that has largely fallen out of use but can still appear in older texts and some technical contexts. The symbol ℧ is sometimes used to denote the mho, and the term itself may be encountered in equipment labels, schematics, or literature from earlier eras.

History and usage: The mho was widely used in the early to mid-20th century in electronics and electrical engineering. As SI units became standardized, the siemens superseded the mho in most modern documentation. Today, the mho is mainly of historical interest or appears in discussions of older devices and publications. The SI unit for conductance remains the siemens, defined as 1 ampere per volt.