electric vector potential: F ( r , t ) = ε ε 0 4 π π ∫ ∫ Ω Ω M i ( r ′ , t ′ ) | r − − r ′ | d 3 r ′ , where F at point r and time t is calculated from magnetic currents at distant position r ′ at an earlier time t ′ . The location r ′ is a source point within volume Ω that contains the magnetic current distribution. The integration variable, d 3 r ′ , is a volume element around position r ′ . The earlier time t ′ is called the retarded time, and calculated as t ′ = t − − | r − − r ′ | c .