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A Gramme machine or Gramme dynamo is kind of electric dynamo named for its Belgian/French inventor, Zénobe Gramme. Gramme demonstrated this apparatus to the Academy of Sciences in Paris in 1871. The Gramme machine used a series of thirty armature coils, placed inside a revolving ring of soft iron. The coils are connected in series, and the junction between each pair is connected to a commutator on which two brushes run. The permanent magnets magnetize the soft iron ring, producing a magnetic field which rotates around through the coils in order as the armature turns. This induces a voltage in two of the coils on opposite sides of the armature, which is picked off by the brushes. Earlier electromagnetic machines passed a magnet near the poles of one or two electromagnets, creating brief spikes or pulses of DC resulting in a transient output of low average power, rather than a constant output of high average power.

With enough coils, the resulting voltage waveform is practically constant, thus producing a near direct current supply. This type of machine needs only electromagnets producing the magnetic field to become a modern generator.

During a demonstration at an inventors' fair in 1873 Gramme accidentally discovered that this device, if supplied with a constant-voltage power supply, will act as an electric motor. The Gramme Machine was the first powerful electric motor useful as more than a toy or laboratory curiosity. Today the design forms the basis of nearly all DC electric motors. Gramme's use of multiple commutator contacts with multiple overlapped coils, and his innovation of using a ring armature, was an improvement on earlier dynamos and helped usher in development of large-scale electrical devices.

• Electricity museum: early motors

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