Acronym for transfer inductor. A device that consists of a magnetic core carrying several windings. The state of magnetic flux density in the core is controlled by a fixed alternating current in one of the windings. This current is sufficiently large to cause saturation of the core. Small variations in the current of one of the other windings – the signal winding – then cause large variations in the power in another circuit coupled by another winding – the power winding. The device thus operates by magnetic modulation and is used in control circuits, such as lighting circuits, and, particularly, in aircraft.

Variations of the current in the signal winding are caused by the control circuit and must be slow relative to the frequency of the alternating current in the supply circuit; frequencies up to about 2 kilohertz are possible and therefore control of signals in the lower audiofrequency range is available. The controlled signal may be output directly from the power winding.