A wire circling a magnet in mercury started a century of electric motors
Down in the Royal Institution's basement on 3 September 1821, Michael Faraday hung a wire into a pool of mercury with a magnet in it. When current flowed, the wire swept around the magnet. He then mailed pocket-sized copies of the gadget to colleagues abroad so they could watch it spin.
Faraday's trick rested on discoveries made months earlier. Alessandro Volta's battery of 1799 had made steady currents possible, and in 1820 Hans Christian Ørsted found that a current produces a magnetic field able to push a magnet. Within weeks André-Marie Ampère had written down a law for the force between currents and magnetic fields. Even before any of that, the Scottish monk Andrew Gordon and Benjamin Franklin had built little motors in the 1740s and 1750s that ran on static electricity, though the high voltages they needed kept them impractical.
Turning a curiosity into a machine required a way to keep the rotation going. The Hungarian physicist Ányos Jedlik solved that with a commutator, a rotating switch that flips the current every half turn, and by 1828 he had a device with stator, rotor and commutator, the three core parts of later DC motors. William Sturgeon in England produced the first commutator motor able to drive machinery in 1832. In the United States, Thomas and Emily Davenport built one that reached 600 revolutions per minute and ran a printing press, patented in 1837, but expensive batteries bankrupted them. Moritz von Jacobi's motor of 1834 set a power record, and his 1838 version carried 14 people across a wide river in an electric boat.
Practical success came with better armatures. Antonio Pacinotti described a ring armature in 1864, Zénobe Gramme reworked it in 1871 into the first commercially successful DC machines, and Siemens and Halske swapped in a drum rotor in 1872. At the 1873 Vienna World's Fair Gramme accidentally showed that the machines were reversible, linking two of them up to 2 kilometres apart so that one generated power and the other ran as a motor.
That reversibility still matters. A generator is mechanically the same device run backwards, which is why traction motors can recover energy through regenerative braking. The biggest modern motors, used for ship propulsion, pipeline compression and pumped storage, exceed 100 megawatts.
Source: Electric motor
