Power flows from negative to positive
negative charged electrons move toward the positive
So you are cutting off the Power using a N channel
And a a 1 inch piece of 18 gauge wire would read 10 Millivolts @12 volts 10 amp and and put out 100 Milliwatts of heat I'm sure it's not going to get hot you'll have to scale it with a opamp.
I have a DVM it's not a cheapo and it reads across a 3 inch piece of wire it's 14 gauge