And.... Here is the rant, from 2006. It was written for the vintage radio group - so "radio" is used instead of "about anything mains-powered with capacitors".


RANT WARNING RANT WARNING RANT WARNING
Well, I might spare you, IF: you have voltage _and_ current meters for
this variac that are capable of accurately representing 0.01A and 2-3V
AC. If you have this capacity and use it *every* time you use a variac,
then ignore the following.

Otherwise, a variac is worse-than-useless for radio work, fit only for
dimming lights without excessive rF noise. I cannot think of a single
use for them (without meters) that a dim-bulb tester will not do with
equal accuracy, far greater safety and less cost (of course). The
typical urban-myth uses for a variac:

a) Apply power to a radio slowly to see how it starts without letting
out the 'magic smoke': Yeah, right. How would you know unless you
either see sparks (it's too late then), or have a current meter on the
thing to see that it is pulling 65 watts when it really only should be
30. A radio might even work just-fine-and-hum-free as far as ears are
concerned, but if it is pulling those extra 5-10 watts, that is all
being expressed as heat somewhere, usually in an unobtaium part. Can
you say *POOF*?

b) Reforming caps: Unless a radio has a SS rectifier, this is simply
silly. A tube rectifier won't pass current until about 75% or-so of
rated filament voltage. So, hitting the caps with 75% of B+ hardly is a
slow reform.

c) Running radios at the "proper" voltage: Great, until they leave the
bench.

d) Dimming lights... Yep, that's what unmetered variacs are best at.
Other thanbas ballast and as doorstops.

END RANT

In all seriousness, unmetered variacs are silly, stupid, dangerous
items for electronics use, and for any of several reasons. Spend the
money where it will do some good, such as an isolation transformer. If
you have one of those, then look for a metered variac, or the meters to
add to the one you have.