Mid Plains Prospectors
Technical Information

History of Metal Detectors
You can use electricity to make magnetism and magnetism to make electricity.  A fantastically clever Scottish physicist named James Clerk Maxwell (1831–1879) summed all this up in the 1860s when he wrote out four deceptively simple mathematical formulas (now known as Maxwell's equations).  One of them says that whenever there's a changing electric field, you get a changing magnetic field as well.  Another says that when there's a changing magnetic field, you get a changing electric field.  What Maxwell was really saying was that electricity and magnetism are two parts of the same thing: electromagnetism.  Knowing that, we can understand exactly how metal detectors work.

Metal detectors apparently date back to the shooting of US President James A. Garfield in July 1881.  One of the bullets aimed at the President lodged inside his body and couldn't be found.  Telephone pioneer Alexander Graham Bell promptly cobbled together an electromagnetic metal-locating device called an induction balance, based on an earlier invention by German physicist Heinrich Wilhelm Dove.  Although the bullet wasn't found and the President later died, Bell's device did work correctly, and many people credit it as the very first electromagnetic metal locator.

Portable metal detectors were invented by German-born electronics engineer Gerhard Fischer (which he also spelled "Fisher") while living in the United States, and he applied for a patent on the idea in January 1933.  He called his invention the Metalloscope—a "method and means for indicating the presence of buried metals such as ore, pipes, or the like".  The same year, he founded Fisher Research Laboratory, which remains a leading manufacturer of metal detectors to this day.  Dr Charles L. Garrett, founder of Garrett Electronics, pioneered modern, electronic metal detectors in the early 1970s.  After working for NASA on the Apollo moon-landing program, Garrett turned his attention to his hobby—amateur treasure hunting—and his company revolutionized the field with a series of innovations, including the first computerized metal detector featuring digital signal processing, patented in 1987.


What make a metal detector buzz when you sweep it over buried treasure?  Why is it important to keep the detector moving?
  1. A battery in the top of the metal detector passes electricity down through the handle to the transmitter coil (red) at the bottom.
     
  2. When electricity flows through the transmitter coil, it creates a magnetic field all around the coil.
     
  3. If you sweep the detector above a metal object (yellow), the magnetic field penetrates right through it.
     
  4. The magnetic field makes an electric current flow inside the object.
     
  5. This flowing electric current creates another magnetic field all around the object. The magnetic field cuts through the receiver coil (blue) moving about up above it. The magnetic field makes electricity flow around the receiver coil and up through the receiver circuit to a loudspeaker that beeps to alert you you've found something.