Rotating copper disc and magnet used for demonstrating Arago's motion, 1889.
© Science Museum / Science & Society Picture Library
In 1820, the French physicist Francois Arago (1786-1853) discovered the production of magnetism by electricity. Arago devised an apparatus consisting of a copper disc mounted horizontally on a central vertical spindle, with a pivoted compass needle mounted separately above the disc. When the disc is spun, using a belt and pulley, the needle follows it. If the rotation of the disc is then reversed, the needle slows, stops and then also moves in the reverse direction. This effect, known as Arago's motion, is due to eddy currents in the disc, although Arago was not able to discover this. His work on electromagnetism was instrumental in inspiring Michael Faraday to make his discoveries in the same field. Made by Harvey and Peak.