A Variac looks like a sci-fi laboratory prop. For some reason, I have three of them in my shop, and yes, one is just a book end. But they do have a useful purpose. Variac is a generic trade name ...

Turn on the variac after making sure the light bulb is connected to the variac properly. Slowly turn the knob to increase the voltage, notice how the color of the filament changes. Return it to zero ...