I have a heating coil (actually an e-cigarette vaporizer) that I soldered two 10cm wires to. If I hook it up to a regulated 5V power source and measure the voltage over the coil, it measures about 4.07V. I find this puzzling because I can't figure out where the remaining 0.93 volts went. I'm measuring straight on the point where the wires connect to the power source leads. If I take away the coil, the leads measure exactly 5V.
I also have a 9V battery that, when measured, still has about 7.47V. If I connect the coil to that and, again, measure on the point where the wires connect to the battery leads, I measure only 2.39V over the coil, meaning 5,08V has somehow disappeared.
How is this happening? Is a heating element like this some sort of non-ohmic resistor? What exactly is going on here?