In the microscopic formulation of electromagnetism, where there is no concept of an H field, the vacuum permeability μ 0 appears directly (in the SI Maxwell's equations) as a factor that relates total electric currents and time-varying electric fields to the B field they generate. In order to represent the magnetic response of a linear material with permeability μ, this instead appears as a magnetization M that arises in response to the B field: M = ( μ μ 0 − − 1 − − μ μ − − 1 ) B . The magnetization in turn is a contribution to the total electric current—the magnetization current.