The 12 Volt car adapter is a very popular source of DC power so its worth knowing more about its parameters.
20-25mm in diameter and 50mm long.
Positive contact at the tip and negative contact on the side.
The negative side contacts are usually attached to springs to hold the device secure.
Frequently one of the negative side contacts may be not electrically connected and simply to help hold the device in place.
Under the positive contact there may be a spring and a fuse, which is normally a 2Amp fuse.
Some adapters in this format may support 24 volts as some truck electrical systems are 24 volt.