wiring
I don't even know what such a part would be called, but here's my problem: I have a bunch of electrical equipment (sensors, relays, etc) on one side of a piece of stainless steel sheet metal. I have a data acquisition card on the other side of said sheet metal. I need to connect the equipment to the DAQ through the sheet metal. I would like to use screw terminals, if possible.
Does a thing like that exist, and what would it be called?
Since you say you don't want a highly technical solution, then I suggest you simply buy a ready made LED strip. This way you don't have to worry about current limiting (which is absolutely needed), it's done for you.
eBay sell thousands of these, but probably the best place to get them is Adafruit - their RGB Strips are likely to be half decent (they sell other versions too, have a browse), and they have a good tutorial on using them.
Wiring Diagram
Below is an example of wiring (from the tutorial above) using an N-Channel power MOSFET such as the STP16NF06 (gate pin 1, drain pin2, source pin 3):
UPDATE:
On rereading it's not obvious where you are connecting this or what it is you are connecting.
If this is a headset cord (as it seems to be) then what I said about voltages is wrong.
What are you connecting to what?
OLDER:
Nothing makes sense.
If this is to a "central Office" you should have 50V DC somewhere.
If to a PBX possibly 25 VDC or lower.
1,3 = 4.2V and 3,1 = 395 mV suggests that you are grounding the signal with your scope ground in the second case. You need a differential probe or isolated scope or measurements relative to ground.
Connecting ground to one wire will unbalance a feed circuit and allow induced noise from all over. Balanced load or feed must be used.
Use a magnetic earpiece of around 1000 Ohms or a high impedance one with a say 470 Ohm resistor across it. Connect to 2 wires at a time and listen for dial tone. Go from there.
Best Answer
In general, electrical connectors that penetrate bulkheads, while maintaining vacuum, pressure, etc. and preventing shorting of the signal are called "feed throughs" and there are many types. Here a particularly fancy and probably very expensive version. They need not be fancy or expensive though.