In the above, μ 0 {\displaystyle \mu _{0}} is the permeability of free space, c {\displaystyle c} is the speed of light, and σ 0 {\displaystyle \sigma _{0}} is the electrical conductivity of the material in question. In case of a plasma, this is the conductivity due to Coulomb or neutral collisions: σ 0 = n e e 2 m e ν c {\displaystyle \sigma _{0}={\frac {n_{e}e^{2}}{m_{e}\nu _{c}}}} , where