unit of electrical conductance. In the case of direct current (DC), the conductance in siemens is the reciprocal of the resistance in ohms (ohm) (S = amperes (ampere) per volts (volt)); in the case of alternating current (AC), it is the reciprocal of the impedance (electrical impedance) in ohms. A former term for the reciprocal of the ohm is the mho (ohm spelled backward). It is disputed whether the siemens was named after the German-born engineer-inventor Sir William Siemens (Siemens, Sir William) (1823–83) or his brother, the electrical engineer Werner von Siemens (Siemens, Werner von) (1816–92).