abmho

An abmho is a unit of electrical conductance in the centimeter-gram-second (CGS) electromagnetic system. One abmho equals 109 mhos, or equivalently, 109 siemens in modern SI units. It measures how readily an electrical material or component allows current to flow, the reciprocal of resistance.

The term comes from "ab-" (short for "absolute") and "mho", which is "ohm" spelled backwards. The mho itself is the reciprocal unit of the ohm (resistance); one mho equals one siemens. The "ab-" prefix in CGS denotes the electromagnetic unit, scaling the base mho by a factor of 109. This awkward nomenclature reflects the historical development of electromagnetic units before SI standardization.

The abmho appears almost exclusively in older electrical engineering literature, particularly in papers and references from the early-to-mid 20th century when CGS was standard in many laboratories. The same measurement in SI units is simply expressed in siemens (S) or microsiemens (μS), which are far more intuitive and universal today. A conductance of one abmho equals one gigasiemens (109 S), a value encountered only in highly specialized, low-resistance applications such as superconductors or plasma physics.

Working with abmhos today is chiefly a matter of unit conversion when reading historical data or specialized theoretical work. The conversion factor is straightforward: divide abmhos by 109 to obtain siemens, or multiply by 109 to convert siemens to abmhos. Most modern instruments, software, and standards documentation use siemens exclusively, making the abmho functionally obsolete in practical electrical engineering.