You are here: Home page > Gadgets > Metal detectors
Advertisement

Soldier sweeping roadside for hidden weapons using a Garrett metal detector

Metal detectors

by Chris Woodford. Last updated: July 12, 2014.

Bleep bleep! Bleep bleep! Is there anything more exciting than discovering treasure? Millions of people all around the world have fun using metal detectors to uncover valuable relics buried underground. Exactly the same technology is at work in our military and security services, helping to keep the world safe by uncovering guns, knives, and buried mines. Metal detectors are based on the science of electromagnetism. Let's find out how they work!

Photo: This US Marine is using a Garrett metal detector to sweep for hidden weapons. Photo by Tyler Hill courtesy of US Marine Corps.

When magnetism met electricity

James Clerk Maxwell

If you've ever made an electromagnet by wrapping a coil of wire around a nail and hooking it up to a battery, you'll know that magnetism and electricity are like an old married couple: whenever you find one, you'll always find the other, not very far away.

We put this idea to good practical use every minute of every day. Every time we use an electric appliance, we're relying on the close connection between electricity and magnetism. The electricity we use comes from power plants (or, increasingly, from renewable sources like wind turbines) and it's made by a generator, which is really just a big drum of copper wire. When the wire rotates at high speed through a magnetic field, electricity is "magically" generated inside it—and we can harness that power for our own ends. The electric appliances we use (in everything from washing machines to vacuum cleaners) contain electric motors that work in precisely the opposite way to generators: as electricity flows into them, it generates a changing magnetic field in a coil of wire that pushes against the field of a permanent magnet, and that's what makes the motor spin. (You can find out much more about this in our article on electric motors.)

In short, 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.

Photo: The brilliant physicist James Clerk Maxwell. Public domain photo by courtesy of Wikimedia Commons.

How electromagnetism powers a metal detector

Walk-through airport metal detector

Photo: This advanced walk-through detector developed at Pacific Northwest National Laboratory uses wave imaging to detect plastic and ceramic weapons not picked up by conventional metal detectors. Photo by courtesy of US Department of Energy.

Different metal detectors work in various different ways, but here's the science behind one of the simpler kinds. A metal detector contains a coil of wire (wrapped around the circular head at the end of the handle) known as the transmitter coil. When electricity flows through the coil, a magnetic field is created all around it. As you sweep the detector over the ground, you make the magnetic field move around too. If you move the detector over a metal object, the moving magnetic field affects the atoms inside the metal. In fact, it changes the way the electrons (tiny particles "orbiting" around those atoms) move. Now if we have a changing magnetic field in the metal, the ghost of James Clerk Maxwell tells us we must also have an electric current moving in there too. In other words, the metal detector creates (or "induces") some electrical activity in the metal. But then Maxwell tells us something else interesting too: if we have electricity moving in a piece of metal, it must create some magnetism as well. So, when you move a metal detector over a piece of metal, the magnetic field coming from the detector causes another magnetic field to appear around the metal.

It's this second magnetic field, around the metal, that the detector picks up. The metal detector has a second coil of wire in its head (known as the receiver coil) that's connected to a circuit containing a loudspeaker. As you move the detector about over the piece of metal, the magnetic field produced by the metal cuts through the coil. Now if you move a piece of metal through a magnetic field, you make electricity flow through it (remember, that's how a generator works). So, as you move the detector over the metal, electricity flows through the receiver coil, making the loudspeaker click or beep. Hey presto, the metal detector is triggered and you've found something! The closer you move the transmitter coil to the piece of metal, the stronger the magnetic field the transmitter coil creates in it, the stronger the magnetic field the metal creates in the receiver coil, the more current that flows in the loudspeaker, and the louder the noise.

So thank you, James Clerk Maxwell, for helping us see how metal detectors work—by using electricity to create magnetism, which creates more electricity somewhere else.

How metal detectors work

Animation showing how metal detectors work

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 activates the transmitter circuit (red) that passes electricity down through a cable in 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 it.
  3. If you sweep the detector above a metal object (such as this old gray spanner), the magnetic field penetrates right through it.
  4. The magnetic field makes an electric current flow inside the metal 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 into the receiver circuit (blue) at the top, making a loudspeaker buzz and alerting you you've found something.

Where are metal detectors used?

Soldiers using a Garrett metal detector

Metal detectors aren't just used to find coins on the beach. You can see them in walk-through scanners at airports (designed to stop people carrying guns and knives onto airplanes or into other secure places such as prisons and hospitals) and in many kinds of scientific research. Archeologists often frown on untrained people using metal detectors to disturb important artifacts but, used properly and with respect, metal detectors can be valuable tools in historic research.

Photo: This wand-type detector, called a SuperScanner and made by Garrett Metal Detectors, is being used to check visitors to a medical clinic in Afghanistan. It runs off a built-in 9-volt battery that provides about 60 hours of continuous operation. If you find metal, the detector lets you know with a combination of flashing LED lights and a warbling noise. It's 42cm (16.5 in) long and weighs 500g (17.6 oz). Detectors like this cost about $200 (£100). Photo by Christopher Admire courtesy of US Army.

Who invented metal detectors?

Alexander Graham Bell's induction balance metal detector being used to locate a bullet following the attempted assassination of President James Garfield.

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.

Artwork: Left: Find that bullet! This sketch by William A. Skinkle, from Frank Leslie's illustrated newspaper of August 20, 1881, shows rather a lot of doctors (!) using Bell's induction balance to find the bullet lost in the President's body. The room on the left contains the equipment, on the table-top, which is labelled "interrupter," "condenser," and "battery" (the boxes at the back of the table). You can just make out wires that stretch around the bottom of the picture through to the President's bed on the right. Presumably Alexander Graham Bell is the bearded man talking on the telephone on the right? Courtesy of US Library of Congress.

A patent drawing of the Gerhard Fischer (Fisher) portable metalloscope metal detector from 1937.

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"—and you can see it in the drawing here. 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.

Artwork: Right: The Metalloscope patented by Gerhard Fischer (Fisher) in 1937, which I've colored to make it easier to follow. The transmitter coil is in the red box at the front; the receiver coil is in the blue box at the back. The transmitter uses inaudible 30,000 Hz signals; the receiver sends out audible signals (with a frequency of about 500 Hz) to headphones, as in a modern metal detector. The transmitter and receiver coils are mounted at right angles to one another so the receiver doesn't pick up signals directly from the transmitter. Artwork courtesy of US Patent and Trademark Office.

Find out more

On this website

On other sites

Books

Patents

If you're interested in technical details, check out these patents:

Sponsored links

Please do NOT copy our articles onto blogs and other websites

Text copyright © Chris Woodford 2008, 2012. All rights reserved. Full copyright notice and terms of use.

Follow us

Rate this page

Please rate or give feedback on this page and I will make a donation to WaterAid.

Save or share this page

Press CTRL + D to bookmark this page for later or tell your friends about it with:

Cite this page

Woodford, Chris. (2008) Metal detectors. Retrieved from http://www.explainthatstuff.com/metaldetectors.html. [Accessed (Insert date here)]

More to explore on our website...

Back to top