A dynamo is a type of electric generator that uses a commutator to produce an electric current. It essentially operates as an electric motor in reverse, so instead of drawing current, it induces it as the commutator spins inside a magnetic field.
The dynamo was first demonstrated in 1832 by Hippolyte Pixii. He designed a machine according to a principle articulated earlier that year by Michael Faraday, which stated that an electrical conductor encircling a magnetic field will produce an electromotive force, a principle now known as Faraday's Law.
In practical terms, this means that if mechanical power is applied to turn a spindle wrapped with a conducting coil inside a shaft lined with magnets, an electric current will be generated. The mechanical force can be supplied by hand-turning a crank, for example, as in the case of Pixii's first machine. The first industrial dynamos of the late 19th and early 20th centuries were powered by coal-burning steam engines or by water impounded by dams and forced to flow through turbines. The resulting direct current could be used to power machinery or lights. Though practical for its time, the dynamo is rather inefficient and has been replaced in industry by more efficient generators such as alternators.