Thus we have reduced the problem of modeling the current, J to two (hopefully) easier problems — modeling the free current, J free and modeling the magnetization and polarization, M μ μ ν ν . For example, in the simplest materials at low frequencies, one has J free = σ σ E P = ε ε 0 χ χ e E M = χ χ m H where one is in the instantaneously comoving inertial frame of the material, σ is its electrical conductivity, χ e is its electric susceptibility, and χ m is its magnetic susceptibility.