Electrical conductivity is the proportionality factor relating the current that flows in a medium to the electric force field that is applied. It is a measure of the ability of electrical charge to move through that material. Resistivity is the reciprocal of conductivity. The units of conductivity are Siemens per meter (S/m). The practical unit is milliSiemens per meter (mS/m). Because Siemen, the unit of conductance, is the reciprocal of the Ohm, the unit of resistance, the units of conductivity are sometimes given as mhos/meter or millimhos/meter. Resistivity is the inverse of conductivity ( ρ = 1 / σ ). The units of resistivity are Ohm meters (Ωm).