I have a DC jack with three terminals. Terminal 1 is tip (+), terminal 3 is sleeve (-), and terminal 2 is a switch, my understanding of which is such that if there is no adapter plugged into the jack, terminals 2 and 3 are connected.
When I plug a DC adapter into the jack, I get the voltage drop I would expect across terminals 1 and 3, about 9V. However, there is zero voltage between terminal 2 and both 3 and 1. How is it possible that there is no voltage drop between either? Shouldn't there have to be a 9V drop between terminal 2 and 3 if there's 0V between 1 and 2?