Hi, I'm new to Physics Forum and I can figure out what it seems should be an easy problem. I have two DC power supplies both of which are putting out 24vdc. In my mind I should be able to test for potential difference between the (-) of one and the (+) of the other and read how many volts they are away from each other. If I do that test I get zero difference in voltage. How is this possible? How can both negatives be the same voltage as the positive on the other power supply?