Generator and motor

In 1821 Michael Faraday builds a wire that rotates around a magnet, a primitive motor. In 1831 he shows the reverse: moving a magnet near a coil produces current, the generator. Joseph Henry does closely related work in the United States.

The contribution is a two-way door. Motion becomes electricity. Electricity becomes motion. A power station is the first direction, spun by steam or water. A robot joint is the second, a motor taking current and making a turn. Remove this pair and you can have a battery and still have no factory and no arm.

The machines of 1831 are demonstrations, not products. It takes the rest of the century to make generators large and motors cheap. The principle does not change.

This is why the timeline puts the generator before the telegraph's maturity, the bulb, and the robot. They are all customers of this trick.