Somehow I’ve managed to make it ~15 years without really understanding the multiple lead ports found on a standard multimeter. At a bare minimum you’ll see COM, V/Ω, and A. COM is the easy one, as this is always attached to the black lead. I’ve always used my meter to sense voltage and/or resistance, so I’ve never connected the red lead to A.
I learned my lesson today when I grabbed a random meter from the lab at school. Someone had been noodling with the lead attachment, so COM and A were connected. When I tried to test AC voltage coming out of the wall, I got a big spark and fried the extension cord I was testing. This happened even though I had the dial set to the correct AC range.
So what happened? I found it difficult to track down the answer online, but I finally came up with this page. Near the bottom you’ll see an image that describes what I did:
Sensing current (A) requires that the meter provide no or very low resistance. Makes sense – if the meter introduced resistance, you wouldn’t get an accurate current reading. This feature can also be dangerous; turn the dial back to V/Ω, but forget to use the correct lead ports, and you’ll short the circuit through the meter. Many meters have safety features to help remind you not to do this, but cheap meters lack these fail safes. Fair warning!