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enter image description here

so I need to put a switch in here, but all wire are black so i dont know which wire will go where. just to give a context, this set of wires WERE used to turn on/off 3 light bulbs. so the 3 wires will go to one side of the switch legs, and the other goes wire to the other side of the switch legs. now, one of the light bulb is dead now(no more fixtures installed, just the wire with electrical tape in it). so in reality, i will just need to utilize 3 wires in the picture.

so, i measured the voltages in the individual wires. 2 of the wires has ~100 volts(top wires), 1 wire has 0 Volts(bottom left) and the last wire has ~30 Volts(bottom right). when i said i measured, what i did was put the red probe of the multimeter on the wire, and the black probe of the multimeter on the tiny metal at the bottom of the picture.

now for my questions, am i right in assuming that:

  1. the 0 volt wire(bottom left) is the dead fixture(so i dont need to bother with it since it is dead anyway)
  2. the top wires(both have ~100 volts), goes to one side of the switch legs and bottom left wire (~30 Volts) goes to the other side of the switch

irrelevant question

  1. what would happen if I tap the probes of the multimeter in the wires(red goes to one wire and black goes to other wire)? is that safe?

EDIT

i forgot to say that I touched the two bottom wires together and nothing happened.

  • 1
    What do you mean by approximately 100 V? In the US (are you in the US?) you should be within 10% of 120V. – tnknepp Oct 12 '20 at 11:01
  • I would look into getting the water leak fixed. There's no reason in the world that a box in your wall should be that rusted, or even rusted at all. That's horrific! Additionally, is there a 2nd switch somewhere that controls these lights? It's looking like you've got a 3-way (US, 2-way, ROW) switch set up there. – FreeMan Oct 12 '20 at 12:48
  • @FreeMan There may be no water leak. I had a similar-looking outlet box in a house we were renting. It was on an exterior wall (house was built of cinder block, coastal Virginia). The walls were so poorly sealed that the humid air would come inside and condense inside the walls. Good conditions for mold and corrosion. Oh, the reason I found this rusty old box was because the conductors were so corroded it almost started a fire (i.e. started smoking and the receptacle melted). Possible more than one issue in this wall. – tnknepp Oct 12 '20 at 13:16
  • @tnknepp in our country we have 220 V coming from the outlet. plus i did some more experimenting. i plugged an extension cord in an outlet and bring it near the light switch. i tapped the probes on the extension cord, and it gave me a reading of ~220 V. then i remove the black probe off the extension cord, and tapped it on the little metal at the bottom of the pic. the reading i got is almost the same as the reading on the top wires in the picture(which is ~100V) – DarkInHere Oct 13 '20 at 14:29

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