It's not an actual polarity you want; remember, this is ac. The
"polarity" applies to which is hot and which is neutra.
Arbitrarily assign one as Hot (H); use the wire to the hot connection on
the fixture as H:
Find an Earth Ground (receptacles are probably earthed).
H - N 120V
H - Earth 120V
N - Earth 0V
If H - N = 0 then N - Earth should be 120V, meaning they are reversed.
Switch the wires with each other.
Assumption: Neutral and Earth are ONLY joined back at the breaker box
and nowhere else!
Validate Earth connection by turning off all power (Mains). Use your
ohmmeter to measure the resistance between Neutral and Earth. It should
only be a few ohms, depending on the length of wire involved, or maybe
SWITCH YOUR METER OFF OHMS AS SOON AS DONE, so you don't accidentally
try to measure voltage with it.
The reason to turn off all power is that, should the polarity be
wrong, you will fry your ohmmeter. I always check first for voltage and
only after knowing it's not 120 V ac do I switch it to ohms. Then as
soon as I finish ohmming thngs out, I put it back to measure ac voltage.
That would be my quick suggestion also. Find a ground somewhere.
connect one side of your meter or one side of light bulb to that. With
lights switched on.
Any wire wire that has potential to that ground is most likely the
live (today black) wire and is either a direct live from the supply
fus/breaker or a switched live.
The wire that has zero or a very low potential to that ground
(although even cheap digital multimeters can be deceiving) is most
likely the neutral (white wire).
Don't mess around with readings of 30 or or some odd voltage on wires
which are disconnected or switched off. In fact a light bulb is often
a better tester than a meter that can pick up odd induced voltage from
other circuits or wires!
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