What is a convenient and safe way to send some low voltage current through wire for testing and identifying purposes? E.g. awhile ago, I wired a two speaker setup behind drywall with one input box from which two sound cables go on either side of the room, each terminating in its own box. Assuming I didn't label wires with electrical tape, what would be an easy way for me to send some current to see on which side it ends up. This is just hypothetical and could be any other similar tests, e.g. if current goes from one side to the other suspecting wire breakage or impedance in between etc. I'm guessing some low voltage device would be safer and easier to hook up than real current.
Yes! For goodness sake, do NOT put any commonly available potential (ie 120V) on the lines, they make very easy to use tools for this. Most sources in your house will be voltage sources, vice current sources, so how much current results is a factor of the resistance in the line. Because these speaker wires will have low resistance when you find both ends, you will get a LARGE current in the lines which could destroy your fuse/breaker or even cause a fire.
What you are describing/seeking, though, is essentially a resistance measurement (ohm reading). Get a multi-meter, the "ohm"/resistance measurement ("upper-case" omega) sends a small current down the wire and measures the voltage, to determine the resistance (using the equation v=ir). In your case, you know you will have found the other end when the measurement of resistance goes to near zero (after starting at off scale, or pegged). I would wrap one of the speaker ends around the black lead (color chosen for no particular reason) then go to each wire on the other end until you get the near 0 reading of resistance.
If the wires are too far apart, make an extender out of an available piece of speaker wire. Simply twist the free piece of wire to one end of the speaker wire on the wall to extend it far enough that your red and black meter leads are long enough to reach both ends. Then continue with the resistance measuring technique described above. Now, use the red lead of the multi-meter to go through each wire until you find the right one. Mark those wires and move on to the next.
A wire tracker is a useful tool for this.
They usually have a few different types of connectors, so they can trace phone, and CATV lines, as well as single wires.
Alternatively, if you're cheap, and you're working on wires where speakers are hooked up. Connecting a 9V battery to the wires, will make the speaker at the other end click. Can't guarantee this won't damage the speaker, and it probably won't work if the speakers have a built in amp.