A synchro or "selsyn" is a type of rotary electrical transformer that is use for measuring the angle of a rotating machine such as an antenna platform. The primary winding of the transformer, fixed to the rotor, is excited by a sinusoidal electric current, which by electromagnetic induction causes currents to flow in three star-connected secondary windings fixed at 120 degrees to each other on the stator. The relative magnitudes of secondary currents are measured and used to determine the angle of the rotor relative to the stator, or the currents can be used to directly drive an electric motor that will rotate in unison with the synchro. In the latter case, the whole device is also called a selsyn (a portmanteau of self and synchronizing).