A saturable reactor was a special form of inductor where the magnetic core could be deliberately saturated by means of a dc current flowing in a control winding. Once saturated, the inductance of the saturable reactor dropped dramatically.

Saturable reactors provided a very simple means of remotely and proportionally controlling the flow of ac current into a load such as an incandescent lamp; the flow of ac was roughly proportional to the flow of dc in the control winding. In addition, because of the particular arrangement of the power windings, the control winding, and the core, the control winding was well-isolated from the ac power.

Because the required inductance to achieve dimming varied with the size of the load, saturable reactors often had multiple taps, allowing a small inductance to be used with a large load or a larger inductance to be used with a smaller load. In this way, the required magnitude of the control current was also held roughly constant, no matter what the load.

The principal disadvantages of saturable reactors was that they were physically large and heavy and somewhat expensive. Today, they have been supplanted by thyristor dimmers using triacs or SCRs.