Current transformers measure power flow and provide electrical inputs to power transformers and instruments. Current transformers produce either an alternating current or alternating voltage that is proportional to the measured current. There are two basic types of current transformers: wound and toroidal. Wound current transformers consist of an integral primary winding that is inserted in series with the conductor that carries the measured current. Toroidal or donut-shaped current transformers do not contain a primary winding. Instead, the wire that carries the current is threaded through a window in the toroidal transformer.