Recently I was working on a house that someone else had rewired. I was trying to install 2 lights above the bathroom sink using the preexisting wiring. To make sure that I knew which wire was for what I went out to my truck and retrieved my digital multimeter.
A multimeter is used to make various electrical measurements, such as AC and DC voltage, AC and DC current, and resistance. It is called a multimeter because it combines the functions of a voltmeter, ammeter, and ohmmeter. Multimeters may also have other functions, such as diode and continuity tests.
I used the beeper function which allowed me to tell which was the incoming line and how it was wired to the light.
Next, I tried to verify that I was getting 115v to the light. I found that I was only getting 50v, so I then used the multi-meter to check the resistance value and found that I had a bad switch.
Once I had tested each of the wires, it was an easy job to wire the lights, and switches.
The multi-meter is an invaluable tool when working with electrical currents.
Always make sure that you use the correct safety equipment, and turn off the power.