Identifying old armored electric wire and mounting new fixture

Question

I am by no means an electrician, my experience does not extend past swapping old light fixtures for new. We just moved into a house that was built in 1907, so I'm not surprised by anything I've come across thus far, however I've heard mixed answers from people I know about this one.
I am trying to just change our dining room light, and upon removing the old fixture, I discovered just 2 wires. Both are covered in a cloth which I've read was common way back in the day. The fixture itself was mounted directly to some joists I'm assuming, as I see no box; only two holes through the wood where each wire comes out. One is wrapped in a thicker material further back.
It would obviously be best to hire a professional to rewire this I'm sure, but is there any clear and present danger to maybe just getting a pancake box and mounting it to the joists? And what's the best way to identify which wire is which, as the old light fixture's wires weren't color-coded either.

manassehkatz-Moving 2 Codidact · Answer

The big thing is figuring out which wire is switched hot and which wire is neutral. Note that it is possible, due to messed up wiring, to have some other combination, such as hot and a switched neutral, which would not be a good thing.
Use a non-contact tester to:

Verify that both wires are "dead" with the circuit breaker (or fuse) off. If not, stop and ask a question. (There can be false positives, but really shouldn't be on a simple fixture connection, as opposed to wires in a box with a bunch of other wires).
Turn on the breaker. Verify that both wires are "dead" with the light switch off. If not, stop and ask a question, as you may have a switched neutral.
Turn on the switch. Identify which wire is hot. This wire is the switched hot. The other wire is the neutral. Mark the neutral wire with white tape.

Turn off the breaker. Wire nut the light fixture hot (typically black, but could be any color except white, green or bare) to the switched hot wire. Wire nut the light fixture neutral (should be white) to the neutral wire. Turn on the breaker, test.

Identifying old armored electric wire and mounting new fixture

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