Michael Faraday first demonstrated the connection between magnetism and electricity by moving a magnet inside a coil of wire. So long as the magnet moved in relation to the coil, an electric current was induced in the wire; when the magnet was stationary, the current ceased. Faraday further suggested that the electromagnetic forces which occurred spread into the area around the wire. The first electricity generator, known as a dynamo, applied these principles with a cranked permanent magnet spinning inside a wire coil. Each time the magnet turned, a current in alternating directions was produced, depending on which pole of the magnet was passing the wire. All electric current available at the time of this discovery was the direct current from the batteries invented by Alessandro Volta, so this alternating current was altered to direct by adding a commutator (switch) to the dynamo design.
The Gramme dynamo, which so intrigued Tesla, improved on previous versions. It was made up of a series of thirty coils, connected in series with a commutator at each connection, placed inside a rotating, magnetized iron ring. It created an almost uninterrupted direct current with the drawback that Tesla noticedsparking at the commutator brushes due to the tiny power disruptions. The dynamo was reversible, a supply of electricity to the coils induced rotation of the magnet, which could be connected to the spindle of a motor. Electrical force could be converted to mechanical force and vice versa.