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I have a hallway light that doesn't light up with the switch is flipped. My first move was to change the bulb. This did not work. I used a multimeter to test the voltage across that socket and I'm getting 80V. I found this surprising because the other bulb sockets I've tested in the house get the expected 120V.
Any thoughts? Is this something where I need a special bulb or do I have a bigger problem that requires some additional work?
I can't disagree with Paul's response especially when it comes to safety. There are certainly a few things that can be considered here and if you are uncertain I would recommend an electrician because what you describe should never be able to happen and now that is has been identified correcting it is crucial.
Given that you can measure near 120, but it is significantly lower than expected, you almost CERTAINLY have a high resistance connection and this is a recipe for fire. It is possible this is a phantom voltage, but should be treated as live until proven otherwise. Intentionally presume fire is imminent, it could happen at any moment. Now that I got the warning "no kidding this is serious" part out of the way:
Possible likely causes that come to mind:
Steps to resolve: [Notice: when opening or touching wire, secure power, turn it back on when ready to test]
In either case, there is and has been a bad connection in your circuit for possibly a long time. You should check all connections from the light back to the breaker panel. High resistance connections draw more current, thus, more heat. So other connections in the circuit could be, or become, compromised.
Correct any and all bad connections, and replace wiring if sheathing is melted. Contact an electrician if any of this is even remotely questionable.
You used a DVM? 80V sounds a lot like phantom voltage, induced by nearby wires. It's a bit like how a crystal AM radio can use the radio waves itself to power a tiny earpiece. This can be picked up by very sensitive digital meters (as contrasted with an analog meter where the voltage must have enough motive force to move the needle). It is not real, usable voltage, and actually means the wire is disconnected.
The easy way to test for phantom voltage is use a less sensitive meter, or put a tiny load on the circuit. The tiny load will quickly extinguish the phantom voltage.
... plug in an incandescent night-light with a switch. If the voltage disappears when you switch the night-light on, that was phantom voltage. Treat it the same as you would a 0 volt reading.
Of course, then you'll need to chase the problem as normal. The good news is, barring nails driven through Romex, 99% of wiring trouble is at the terminations, and Code requires every termination (with rare exception) be inside a junction box which is accessible without deconstructing the building.
It goes without saying that you treat every gun as loa--- I mean every circuit as hot, even when you personally cut the breaker you are sure it is. Things like MWBCs can boomerang back on you.
Once I overhauled the wiring in a factory. The 480V feeder to the buses, and the service entrance wire off the 120V transformers had been stolen. The 1000A 480 service and 480-120 transformer supply knife switches were off. This was way beyond LO/TO, the service wires were gone. I still treated every wire like it was live. You don't want to wire your brain to ever think it's OK to touch copper. This is one reason I pigtail receptacles at a bench instead of hooking them onto screws at the box.
