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Just moved into a 1940s/50s house, very old wiring. I noticed all the outlets in one room indicate reverse polarity when tested with a basic outlet tester. I took one out to examine the wiring, and it seems to be wired correctly.

I then noticed that while the outlet was out of the receptacle, the tester indicated correct installation (hadn't actually done anything with the wires yet). These outlets are ungrounded, with a single neutral and single hot wire. Touching one of the outlet screws to the receptacle causes the tester to flip back to 'reverse hot/neu', pulling it back away returns the tester to correct installation.

Ideas why? Ideas how to fix?

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    Touching it to the box grounds it, even without a ground wire.. That ground connection is needed to report hit/neutral reversed. Accept the tester's advice and swap those connections.
    – keshlam
    May 9 at 19:05
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    When the receptacles are out of the wall, does the tester actually show "correct" (which it shouldn't with no ground) or "open ground"?
    – manassehkatz-Moving 2 Codidact
    May 9 at 19:57

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It may well be reversed. But don't assume it's reversed at that receptacle; the reversing could be at any point along the cable run.

Without some sort of ground connection, it's impossible to say whether an AC supply is reversed or not. There are two wires with 120V between them, and no way to say which is which.

But the neutral is connected to ground. By connecting the outlet screws, there may be enough of a ground reference for the tester to work it out. That ground reference could even be through the walls of the house. A tester will only be using tiny currents.

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