The Siemens is a unit of electrical conductance. It is named after the German engineer Werner von Siemens. The unit was formerly called the mho, and its symbol was ℧ (the reverse of Ω). The siemens is equivalent to the inverse ohm – 1/Ω or A/V – and can be used to express the efficiency of an electrical conductor or device in terms of how much current it can carry for a given voltage drop.

The siemens is also a derived SI unit representing inductance, capacitance, and impedance that are expressed in ohms; hence, one siemens equals one ampere per volt. In general, usage, unless explicitly stated otherwise, the word “siemens” refers to Electrical Conductance Quantities such as reciprocal resistance R-1 (= 1/R), which includes both DC and AC conduction components represented by their respective resistances RDC and RAC. For historical reasons dating back to early 20th century commercial practice in alternating current circuits using two-wire transmission lines carrying sinusoidal currents at power frequencies below approximately 100 Hz., apparent power S (in volt-amperes VA) multiplied by conductivity σ (in siemens S / m) gives active power P (in watts W): P = VASG, where G = σ / 2πf represents linear conductance measured in units of reciprocal length L -1· (S · m)-1.