Recommended · for some experience
Big Dim Bulb Tester with Variac and Isolation Transformer
Why people recommend it
Recommended for Mr Carlson's design of a dim bulb tester, a protective tool for safely powering up old mains equipment during repair and restoration.
Watch for: Mains voltages; check where the isolation transformer belongs
In their words
And here's Mr Carlson's dim bulb tester schematic...
Nick Bolton on electronics.stackexchange.com, Feb 2023. CC BY-SA.