Yep, done that too, But my old Variac isn’t as pretty as that one, though — I scrounged it from a rack panel in a MIL surplus store. The windings are all open, so you have to be real careful handling it. But then, you can see how it works :-)
Incidentally, that Variac came in handy in another context. The band was playing for an outdoor rural festival that was powered by gas generators, and the organizers had run looooong power cables to the stage, so we were getting maybe 105VAC lightly loaded, but whenever the bass player hit a power note, the line dropped to around 95VAC, and as a result, our fancy digital signal processing rack gear would reset. I wired up the Variac (of course I carried it with me; ya never know) as a boost autotransformer and the show went on...