Siemens (unit, $\mathrm{S}={\Omega }^{-1}$)

The siemens, symbol $\mathrm{S}$, is the SI unit of electrical conductance and electrical admittance: $1\mathrm{S}=1A/V=1{\Omega }^{-1}$. A conductance of $2\mathrm{S}$ corresponds to resistance $0.5\Omega$ for an ideal resistor.

The same unit can describe a complex electrical admittance in a sinusoidal experiment; that value includes a reactive part as well as conductance. For a uniform conducting sample, electrical conductivity instead has units $\mathrm{S}{\mathrm{m}}^{-1}$ because the conversion from material response to conductance also involves sample area and length.