I'll insert here (after Ludwik's comment) a standard warning I give to
my students about using transformers:
A VARIAC is not a transformer. It is an autotransformer. If you
don't understand the difference, don't try to improvise anything
using a VARIAC.
The arbitrary grounding of one side of the output of a VARIAC can lead
to exciting results. I know Ludwik did not advocate that procedure,
but one doesn't require a demonstration to believe that molten copper
burns are painful.