Specifically, suppose that one has a current density J 1 that produces an electric field E 1 and a magnetic field H 1 , where all three are periodic functions of time with angular frequency ω, and in particular they have time-dependence exp ⁡ ⁡ ( − − i ω ω t ) . Suppose that we similarly have a second current J 2 at the same frequency ω which (by itself) produces fields E 2 and H 2 . The Lorentz reciprocity theorem then states, under certain simple conditions on the materials of the medium described below, that for an arbitrary surface S enclosing a volume V :