Magnetic Recording/Audiocassette

Before 1820, magnetism and electricity were two completely separate fields of science. Magnetism was associated with the attraction of magnets for iron objects and the use of a compass needle to locate north and south. Electricity was of practical interest in connection with the hazards of lightning. Some scientists experimented with static electricity in the laboratory by rubbing a wool cloth against glass, but no useful applications came about.

In 1821, a Danish physics teacher named Hans Christian Oersted made a remarkable discovery while doing a demonstration for his class. He had made a crude chemical battery by placing strips of copper and zinc into an acid solution. By connecting the two metal terminals with a wire, he provided a path for electric current to flow. A magnetic compass was lying on the table nearby. To his great surprise, Oersted noticed that the compass needle would deflect whenever current flowed through the wire. Apparently, the electric current created a magnetic field around the wire. His discovery was the beginning of electromagnetism, a joining of these two sciences.

Other scientists followed up on Oersted's discovery. For example, it was found that a much stronger magnetic field could be produced by winding the electric wire into a coil. Also, an iron core at the center of the coil intensified its magnetic field even more. Joseph Henry, an American inventor who later became head of the Smithsonian Institution in Washington, D. C., made an electromagnet that was powerful enough to support a load weighing 2,000 lb (908 kg).