where we have used the definition of the current density, and also put σ σ = n e e 2 ν ν m e which is the electrical conductivity. This equation can also be equivalently written as E + v × × B = ρ ρ J , where ρ ρ = σ σ − − 1 is the electrical resistivity. It is also common to write η η instead of ρ ρ which can be confusing since it is the same notation used for the magnetic diffusivity defined as η η = 1 / μ μ 0 σ σ .