Bottle dynamo

A bottle dynamo or sidewall dynamo is a small electrical generator for bicycles employed to power a bicycle's lights.

The traditional bottle dynamo (pictured) is not actually a dynamo at all (which creates DC power), but a low-power magneto that generates AC.

Newer models can include a rectifier to create DC output to charge batteries for electronic devices including cellphones or GPS receivers.

[1] Named after their resemblance to bottles, these generators are also called sidewall dynamos because they operate using a roller placed on the sidewall of a bicycle tire.

When the bicycle is in motion and the dynamo roller is engaged, electricity is generated as the tire spins the roller.