As John pointed out, it's a variable autotransformer. It would have been used for dimming lights or anything else that relies on variable voltage (such as a DC motor). It would not have been used on an AC motor. How do you know that it was still operational if you don't know what it controlled? Look for a largeish bank of lights or a DC motor. Auto transformers are robust and long lived, but they do eventually break down when the insulation on the windings fails. At that point, you'll just get line voltage all the time, no matter where the brushes are on the coil. - Jim Katen, Oregon I couldn't find anyhting which was operated by the powerstat, it could be turned on by a push button switch on the other side of the wall, and the bulb at the bottom right hand corner would get brighter as the dial was turned to the right. I talked to an electrician friend of mine who thought it probably controlled a fan or vent which was no long in place. Thanks for the replies.