Metal Detector

A metal detector is an instrument that senses nearby metal, generally built from a control box, an adjustable shaft, and a pickup coil that signals a find by tone, numeric reading, light, or needle movement. It works by passing alternating current through a coil to create an alternating magnetic field; when conductive metal enters that field it develops its own eddy currents and magnetic field, which a second coil senses as a change. A simple electric conduction device for finding metal appeared around 1830, but the modern electronic metal detector dates to the 1920s: Gerhard Fischer built a portable version in 1925 and received the first patent on an electronic metal detector that year, selling his Fisher branded device to the public from 1931. Shirl Herr of Indiana had actually filed for a hand held Hidden Metal Detector earlier, in February 1924, though his patent was not granted until July 1928, and his device saw practical use on Admiral Richard Byrd's Antarctic expedition of 1933, finding buried objects at depths of up to eight feet.