Metal detectors
by Chris Woodford. Last updated: September 1, 2012.
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 and Defense Imagery.
When magnetism met electricity
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
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.
