If the current is distributed in space with a current den sity j, then the element of current Idl is replaced by dr and the magnetic induction expressed as r B 4丌 l-2 ere the integration is carried over the volume T'occu pied by the currents Remarks: These two expressions for b are equivalent The flux of the magnetic induction B through a surface s is defined as Φ=/sB·da Φ has unit weber