I am not understanding this behavior, and I'm hoping that someone with a lot more experience with wiring than I have will know immediately what's going on.
I have two 1940s-era fabric-clad wires attached to a simple two-pole on/off toggle switch in the wall. There's no ground wire. The switch controls a light on the wall.
I know which breaker in the basement controls it. I can flip the breaker off and then test the wires for power with a voltage tester, and see that it's dead.
So, with the power enabled:
When the light switch is in the OFF position, if I touch the tester prongs to the wires, the tester lights up.
When the switch is in the ON position, if I touch the tester prongs to the wires, the tester does not light up.
That is counterintuitive to me, so I flipped the breaker off, reversed the wires on the switch, and turned power back on.
But the behavior remains the same.
Why does turning the switch ON cut the circuit?