
Metal detectors
Last updated: July 9, 2008.
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. Other popular makes include Tesoro, White's,
Bounty Hunter, Fisher, and Teknetics. 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 have one, you can generally 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 flows magically through 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 formulae (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.
Maxwell in 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 metal field in the
metal, the ghost of James Clerk Maxwell tells us we must also have an
electric field in there too. In other words, the metal detector
creates 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 in
the metal.
It's this magnetic field in 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.
Metal detectors at work

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. Archaeologists often frown on untrained people using metal
detectors to disturb important artefacts 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 and
Defense Imagery.