An electrical component used to transfer electric energy from one alternating-current (ac) circuit to another by magnetic coupling. Essentially it consists of two or more multiturn coils of insulated conducting material, so arranged that any magnetic flux linking one coil will link the others also. This configuration creates mutual inductances between the coils. The mutual magnetic field acts to transfer energy from one input coil (or primary winding) to the other coils, which are then referred to as secondary windings. Under steady-state conditions, only one winding can serve as a primary.