Do you know of anywhere I could learn more about this? Googling "old sparky" just yields articles about electric chairs, even when including keywords such as "rheostat" and "theatre"
That was just the theater's name for it. I imagine other theaters had similar contraptions, and because of the expression they probably came to similar names.
It was at least 40 years old. I would hope they've all been retired.
But at least Old Sparky didn't produce toxic fumes. It was merely a fire hazard.