<P> A dynamo is an electrical generator that produces direct current with the use of a commutator . Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later electric - power conversion devices were based, including the electric motor, the alternating - current alternator, and the rotary converter . Today, the simpler alternator dominates large scale power generation, for efficiency, reliability and cost reasons . A dynamo has the disadvantages of a mechanical commutator . Also, converting alternating to direct current using power rectification devices (vacuum tube or more recently solid state) is effective and usually economical . </P> <P> The word dynamo (from the Greek word dynamis (δύναμις), meaning force or power) was originally another name for an electrical generator, and still has some regional usage as a replacement for the word generator . The word "dynamo" was coined by Werner von Siemens in 1882 . The original "dynamo principle" of W. Siemens meant only the direct current generators which use exclusively the self - excitation (self - induction) principle to generate DC power . The earlier DC generators which used permanent magnets were not considered "dynamo electric machines". The invention of the Dynamo principle (self - induction) was a huge technological leap over the old traditional permanent magnet based DC generators . The discovery of the dynamo principle made industrial scale electric power generation technically and economically feasible . After the invention of the alternator and that alternating current can be used as a power supply, the word dynamo became associated exclusively with the commutated direct current electric generator, while an AC electrical generator using either slip rings or rotor magnets would become known as an alternator . </P> <P> A small electrical generator built into the hub of a bicycle wheel to power lights is called a hub dynamo, although these are invariably AC devices, and are actually magnetos . </P> <P> The electric dynamo uses rotating coils of wire and magnetic fields to convert mechanical rotation into a pulsing direct electric current through Faraday's law of induction . A dynamo machine consists of a stationary structure, called the stator, which provides a constant magnetic field, and a set of rotating windings called the armature which turn within that field . Due to Faraday's law of induction the motion of the wire within the magnetic field creates an electromotive force which pushes on the electrons in the metal, creating an electric current in the wire . On small machines the constant magnetic field may be provided by one or more permanent magnets; larger machines have the constant magnetic field provided by one or more electromagnets, which are usually called field coils . </P>

What type of device is a bicycle dynamo