The devices used for these tests and the test circuit can be seen below in Photo 1 and Schematic 1. You plug it into the wall and it converts the AC voltage to DC voltage at some specified current. If you're like me, you simply can t throw these things away if they still work, even when the item they came with and powered is long gone. You have to have the right power plug to fit the new device. Your new device won't last very long if you apply positive voltage to the negative connection.
Most devices are still center positive but more and more are center negative.
It puts the positive voltage on that long outer barrel and cars are usually chassis negative. You need to make sure there is sufficient current. This is because most transistors and Analog ICs are designed to work within a range of voltages. The digital ICs used today can also run on a range of voltages or there is an internal voltage regulator in the design if they cannot. The big problem comes from electrolytic capacitors, which may be rated at close to the operating voltage of the device.