What is clip on ammeter?

These are usually small circular gauges with a clip on the back.

When clipped onto an electric cable, usually from the dynamo or alternator, it shows the amperage being charged out. It is a quick and reliable unit to see if a dynamo etc is working properly.

As a mechanic I used one many times and found them to be invaluable.

I have absolutely no idea how they work. It's either magic or magnetism caused by the charged electricity.