I am attempting to measure the voltage from my wall outlet. I know all the proper precautions when using the multimeter for measuring voltage from a wall outlet, problem is, when I put the leads into the sockets, my multi meter is reading zero. I have it set to 200V AC (I am in the US so this is sufficient for the standard 120.) Do I need to have something plugged into the other socket? Why is it zero? Thanks a lot.