Hub Dynamo

https://en.wikipedia.org/wiki/Hub_dynamo

Hub dynamo - Wikipedia A hub dynamo is a small electrical generator built into the hub of a bicycle wheel that is usually used to power lights. Often the hub "dynamo" is not actually a dynamo, which creates DC, but a low-power magneto that creates AC. Most modern hub dynamos are regulated to 3 watts at 6 volts, although s...