[Radioroslyn]

By definition what is a variac? Is it a variable auto transformer or is it a transformer with variable secondary? The first not providing isolation and the second providing isolation. The definitions I've read seem to intermix the two.

I was cleaning up in the garage the other and found this in the trunk of my old car. I remember "winning" it at a live auction but didn't remember how BIG it was. Good for 20A.

[Arran]

Terry;
  A true "Variac" is a variable transformer manufactured by the General Radio Company, the rest are just variable transformers. It's sort of like how "Formica" in the U.S, and "Arborite" in Canada became the vernacular for any type of plastic laminate material, or how "Masonite" became the vernacular term for pressed hardboard panels. I have three or four variable auto transformers and I think one is a genuine General Radio "Variac" the rest were made by competitors, one by Ohmite, another is Japanese. Having a 20 amp one is nice, you can have lots of fun experimenting with one of those without having to worry about burning it up, I think my go to one is only 6 amps.
Regards
Arran

[morzh]

Variac is usually an auto-transformer. Actually, Arran answered it. It's like Russians call all SUVs "Jeeps". Or we used to call all refrigerators "Frigidaires".

Then there are isolated output Variacs. Those are also autotransformers with the output going through a transformer. Or could be vice versa (can't be sure - in theory that is possible).