A variac is a transformer used to adjust an AC voltage. The windings are mounted on a toroidal form. Some variacs are actually autotransformers, and only use one winding, whereas others have a seperate primary and secondary wound over each other. The secondary winding is on the outside, and has a movable contact similar to that in a potentiometer. One of the more common uses of a variac is to adjust for changes in AC line voltage, notably on quirky old X-ray equipment where the Line Adjust had to be turned until a meter pointed in the right place to assure correct exposure.