In this video I give a review of what is a variac (a.k.a autotransformer), how one works in general terms, some usages, a teardown and I finish with a demonstration.
You can order one of these nice 20 amp variacs from Circuit Specialists (link below).
They are a great local electronics supplier that I use and they ship worldwide.
Here is their selection of variacs:
And here is the variac I use in the video: