How Do Metal Detectors Work? The Science of Magnetism and Metal

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Magnetism plus metal equals one cool—and sometimes lucrative—hobby.

Armed with metal detectors, curious explorers are finding historical objects every day. For example, Bob Podhrasky, COO, senior vice president, and director of engineering at metal detector manufacturer Garrett, told Popular Mechanics that he used a metal detector to explore the battle site at the San Jacinto Monument where Texas gained its independence.

"They had one area where there was a group of personal artifacts in a line, and they suspected that's where the soldiers surrendered and had to empty their pockets out on the ground," Podhrasky said.

How do metal detectors work? First of all, metals interact with magnetic fields.

"If you have a time-varying magnetic field, that magnetic field inside that metal creates eddy currents," Kamal Sarabandi, professor of engineering at the University of Michigan, told Popular Mechanics. "These are currents that would be circling around the magnetic field. In return, those electric currents that are induced in metallic objects can create their own magnetic field that is detected by… the metal detector."

Each detector has a transmitter that energizes a search coil. A receiving coil detects metal objects, which have eddy currents in them due to the search coil's field.

"These receiving loops are positioned in a particular way that the primary transmitting field is canceled out," Podhrasky said. "And so when the metal detector search coil passes over a piece of metal, the magnetic field is disturbed, and so… it creates a change in energy in the receiver coil."

The transmitter is a simple circuit that usually creates a sine wave oscillating, Podhrasky said. The signal from the receiver goes to a preamplifier, which filters out unwanted frequencies, and then is converted to DC and digitized. The circuit creates an audible sound. Many metal detectors now have LCD displays that show their settings.

Metal detectors can tell the differences between different types of metal based on how the magnetic fields are distorted, how much they are distorted, and how much energy is lost as heat, Podhrasky said.

Podhrasky said he invented a technology called "notching" that allows users to use a microprocessor to select the types of objects they want the metal detectors to observe. "For… the Garrett system, we have a series of squares across the top of the screen. Those squares kind of go the range from the very low conductors on the left to the very high conductors on the right. We can turn the pixels off and on. If the pixel's off, if an object falls in that range, the detector does not make a sound."

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Metal detector coils are available in a variety of sizes. Smaller ones are more sensitive to small objects near the surface while larger ones can detect metal at greater depths, Podhrasky said. Larger coils are heavier than small ones. Coplanar concentric coils work well on ground with less mineral content. DD coils are more optimal for ground with high levels of minerals.

Usually, underwater metal detection is done using pulse-induction metal detectors, Podhrasky said.

"Over the years, it's been a constant development process as electronics have advanced," Podhrasky said. "At one point, metal detectors used to have vacuum tubes and you couldn't do very much with them. They couldn't discriminate, and they weren't very sensitive. But over the years, with the advancement in transistors and microprocessors and the ability to do digital signal processing and low power as the complexity goes up, you get LCDs for display. It's a constant evolution of applying technology to the discipline."

There are many uses for metal detectors. People enjoy exploring outdoor sites such as parks, schools, and beaches to find interesting or valuable objects. Metal detectors are also used for mine removal on former battlefields and for security systems for weapons detection. They are also used to prevent metal pieces from being allowed into processed food.

Medically, one famous use of a metal detector involved Alexander Graham Bell's search for a bullet that wounded the U.S. President James Garfield. (Spoiler alert: It didn't work because Garfield was on a bed with metal springs.)

Inquisitive explorers have found rings, jewelry, and many more items that may have financial value or help reveal the past. However, legal restrictions exist in some countries that are concerned about unwanted excavation.

Technologies have changed as people seek to create ways to evade metal detection. "They manufacture the weapons so there is less metal used, so there are these plastic guns or ceramic guns that are hitting the market," Sarabandi said. "And for the military, they are now using plastic mines as opposed to metallic mines, so the detection of those things is very difficult."

Kat Friedrich is a former mechanical engineer who started out as an applied math, engineering, and physics major at the University of Wisconsin-Madison. She has a graduate degree in science and environmental journalism and has edited seven news publications, two of which she co-founded. She spends her free time learning about dance and functional fitness, reading science fiction, and exploring music events.

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