The siemens (symbol: S) is an SI derived unit of measurement for electric conductance, being the inverse of the ohm (Ω), named after Werner von Siemens.

S = A / V

Mho is a now eccentric name for this unit of electrical conductance, equal to 1 Ω^-1. It is derived by spelling ohm backwards. Its symbol is (the upside-down letter Ω). The term siemens is used universally in science and primarily in electrical applications, while mho is used primarily in electronic applications.

SI electricity units

SI electromagnetism units
Name	Symbol	Dimensions	Quantity
ampere (SI base unit)	A	C / s	Current
coulomb	C	A · s	Electric charge, Quantity of electricity
volt	V	J / C = kg m2 / (s2 C)	Potential difference
ohm	Ω	V / A = kg m2 / (C2)	Resistance, Impedance, Reactance
ohm &middot metre	Ω &middot m	kg m3 / (C2)	Resistivity
farad	F	C / V = C2 s / (kg m2)	Capacitance
farad per metre	F / m	C2 s / (kg m3)	Permittivity
reciprocal farad	F−1	kg m2 / (C2 s)	Elastance
siemens	S	Ω−1 = C2 / (kg m2)	Conductance, Admittance, Susceptance
siemens per metre	S / m	C2 / (kg m3)	Conductivity
weber	Wb	V &middot s = kg m2 / (C s)	Magnetic flux
tesla	T	Wb / m2 = kg / (C s)	Magnetic flux density
ampere / metre	A / m	C / (s m)	magnetic induction
ampere-turns / weber	A / Wb	C2 / (kg m2)	Reluctance
henry	H	V &middot s / A = kg m2 s / (C2)	Inductance
henry per metre	H / m	kg m s / (C2)	Permeability
(dimensionless)	-	-	Magnetic susceptibility