As I mentioned in an earlier post, I keep at least one Variac around at all times as a prop, should a sci-fi / horror movie erupt without warning in my shop. The Variac, besides looking like an electronic torture device, is a specialized type of autotransformer. An automotive ignition coil, the part under your hood that generates the super high voltage needed to fire the spark in a spark plug, is another. I cut one in half with a bandsaw to see how it works.
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 for a variable autotransformer. If that doesn't help explain much, let's look at what a regular transformer is, and how they relate to a collection of vintage arcade games.