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 metal detector to sweep roadside tires for hidden explosives. Photo courtesy of US Department of Defense and Wikimedia Commons.
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.)
Photo: The brilliant physicist James Clerk Maxwell.
Public domain photo by courtesy of Wikimedia Commons.
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.
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How electromagnetism powers a metal detector
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.
Artwork: The modern-style, compact metal detector was invented by Charles Garrett in the early 1970s.
You can clearly see the two coils (which I've colored red and blue). The box (orange) at the top of the handle (green) contains the control circuitry, including a battery (not shown), loudspeaker (24), volume switch (27), sensitivity control (28), and on/off switch (29). This illustration is from Charles Garrett's US Patent 3,662,255, granted in 1972, courtesy of US Patent and Trademark Office.
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
What make a metal detector buzz when you sweep it over buried treasure? Why is it
important to keep the detector moving?
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.
When electricity flows through the transmitter coil, it creates a magnetic field all around it.
If you sweep the detector above a metal object (such as this old gray spanner), the magnetic field penetrates right through it.
The magnetic field makes an electric current flow inside the metal object.
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.
What are the different types of metal detectors?
As we saw up above, magnetic fields are produced by changing electric fields, which oscillate at a particular
frequency. Different frequencies give better or worse results depending on what kind of
metal you're looking for, how deep in the ground you're searching, what kind of material the ground is made from
(sand or soil or whatever), and so on.
Although metal detectors all work in broadly the same way, by converting electricity into magnetism and back
again, they come in three main types. The simplest ones are suitable for all kinds of general-purpose
metal-detecting and treasure hunting. They're called VLF (very low frequency) detectors because they use
a single, fixed detecting frequency typically around 6–20 kHz (generally less than 30kHz).
Photo: This foldable VLF mine detector (a US Army Vallon VMW1) can be used on land or underwater at depths up to 30m (100ft). Photo by Kimberly Trumbull courtesy of US Army, published on Wikimedia Commons.
You'll also come across PI (pulse induction) detectors, which use higher-frequencies and
pulsed signals. They can generally pick things up deeper in the ground than VLF detectors, but they're not as discriminating and
nothing like as commonly used. A third type is known as the FBS (full-band spectrum) detector, which uses multiple frequencies simultaneously—so, in effect, it's a bit like using several slightly differently tuned detectors at the same time.
Photo: Clearing mines. This army mine detector (a CyTerra AN/PSS-14) combines
a super-sensitive, pulsed metal detector and a ground-penetrating radar (GPR) in a single,
handheld unit. It can detect mines with low metallic content and distinguish between mine metal, irrelevant metal clutter, and soil with high metal content. Photo courtesy of US Army published on Flickr under a Creative Commons (CC BY 2.0) licence.
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How deep will a metal detector go?
There's no exact answer to that question, unfortunately, because it depends on all kinds of factors, including:
The size, shape, and type of the buried metal object: bigger things are easier to locate at depth than small ones.
The orientation of the object: objects buried flat are generally easier to find than ones buried with their ends facing downward, partly because that creates a bigger target area but also because it makes the buried object more effective at sending its signal back to the detector.
The age of the object: things that have been buried a long time are more likely to have oxidized or corroded, making them harder to find.
The nature of the surrounding soil or sand you're searching.
The type of detector and the frequency (or frequencies) it's using.
Generally speaking, metal detectors work at a maximum depth of about 20–50cm (8–20in).
Where are metal detectors used?
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 people at a traffic control point in Iraq.
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 Ali Hargis courtesy of Joint Combat Camera Center Iraq and DVIDS.
Who invented metal detectors?
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.
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: 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.
What about nonmetal detectors?
Treasure hunters will always value metal detectors like these because, historically, valuable things were usually made of metal.
But in the world of security, it's no longer enough to rely on metal detectors as our sole line of
defence. The kinds of people who like to smuggle weapons through security, for example, are well aware
that they'll have to walk through metal detectors, and they're likely to try alternatives like ceramic,
plastic, or carbon-fiber knives. Although reputable manufacturers do take pains to ensure there are small metallic parts in
the handles of "non-metallic" knives, for exactly this reason, there's nothing to stop anyone sharpening a piece of plastic to
improvize a knife, as the police have repeatedly
found. How, then, do we detect nonmetallic threats?
One solution adopted by airports is to use millimeter-wave scanners (MMS) to show up metal and nometal objects.
Essentially, they work like safer versions of X ray machines: the waves pass through clothing but
are reflected back by our bodies, and any concealed weapons (metallic or otherwise) show up as pictures on a screen.
X-ray machines use very powerful radiation (with wavelengths roughly in the nanometers or billionths of meters), which can be dangerous if your body absorbs too much of them.
As their name suggests, millimeter-wave scanners use much longer waves that measure 1–10mm (roughly 10 times smaller than the microwaves sent and received by cellphones), which are much lower in intensity, and therefore pose
little or no risk to people's health.
Photo: This early prototype detector developed at Pacific Northwest National Laboratory c.2013 uses millimeter-wave imaging to detect plastic and ceramic weapons
not picked up by conventional metal detectors. It can also be used to make digital body measurements for
the clothing industry.
Photo courtesy of PNNL and US Department of Energy (Flickr).
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Don't want to read our articles? Try listening instead
Code of Practice for Responsible Metal Detecting: Although written for the UK, the sensible guidance here will apply more generally in other countries, but be sure to find out about laws or regulations that apply specifically to your area.
Detecting Items Hidden on a Person or Inside a Body: A brief review of some cutting-edge detector technologies by the US National Institute of Justice, including millimeter-wave (mmW) radar and ultrasound.
Chapter 3: Metal Detection: This useful (though slightly dated) 1999 review comes from a US Department of Justice report on "The Appropriate and Effective Use of Security Technologies in US Schools," and includes a look at walk-through detectors, hand-held metal detectors, and X-ray baggage scanners. [Archived via the Wayback Machine]
Archaeology and Metal Detecting by Alex Hunt. BBC News, February 17, 2011. Can professional archaeologists and amateur metal detectors work happily, side by side?
US Patent 4,709,213: Metal detector having digital signal processing by Robert J. Podhrasky. Patented November 24, 1987 (applied for July 8, 1985). Garrett's first computerized metal detector. The search signals are digitized and processed by a computer chip for more precise searching.
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