Electromagnet, c 1990s.

UNITED KINGDOM - JUNE 12: This electromagnet was made by the workshop staff at the Science Museum, London. In 1825, English inventor William Sturgeon (1783-1850) wound a coil of wire around an iron rod and made one of the first electromagnets. Electromagnets, which are powered by electricity, differ from permanent magnets in that the magnetism can be turned on and off. One terminal of the electromagnet is positively charged and the other is negatively charged, so that when the current is turned on the magnetic field around the wire makes the core magnetic. (Photo by SSPL/Getty Images)