How Do Metal Detectors Work? The Electromagnetic Physics

I spent an entire summer afternoon on a windswept Oregon beach watching an older hobbyist sweep a plastic disc across damp sand. Every few paces, his headphones would squeal, he would drop to his knees with a plastic scoop, and up would come a beer tab, a rusty nail, or an old nickel.

I asked him how the machine grabbed onto gold and silver when neither metal is magnetic.

He tapped the side of his control box and smiled. "It doesn't care about magnets," he told me. "It's building its own little power plants in the dirt."

He was dead right. Most people assume a metal detector is just a glorified junkyard electromagnet on a stick, waiting for iron to tug on it. But test that assumption against basic chemistry and it falls apart. Pure gold will not stick to a refrigerator magnet. Neither will silver, copper, aluminum, or lead. If a metal detector relied on static magnetic attraction, swinging it over a buried 24-karat wedding band would yield complete silence.

So how does a plastic coil hovering five inches above the mud pick out a lost coin with pinpoint accuracy?

It exploits a delicate chain reaction of physics: moving electricity, invisible magnetic bubbles, and swirling miniature electrical whirlpools.

The Search Coil: Two Antennas Locked in a Standoff

The plastic dish at the bottom of the shaft looks like a solid molded saucer, but it is actually hollow. Inside sit two carefully arranged loops of insulated copper wire that never touch.

One is the transmitter coil. The other is the receiver coil.

The transmitter is the agitator. The control box pumps an alternating electrical current through this outer wire loop, reversing direction thousands of times per second—typically between 3 kHz and 30 kHz in standard Very Low Frequency (VLF) machines. Whenever electricity pulses through a coiled conductor, physics demands a consequence: it generates a magnetic field. Because the current constantly switches polarity, the magnetic field expands outward into the earth and collapses back in on itself at that exact same rhythm.

This behavior rests on electromagnetic induction (opens in a new tab), first mapped systematically by Michael Faraday in 1831. As Faraday famously demonstrated, electricity and magnetism are two sides of the same coin. Change one, and you inevitably trigger the other.

Then there is the inner loop—the receiver coil.

Engineers wire the receiver in what is known as an induction balance arrangement. They physically overlap the two coils in precise concentric circles or an intersecting "Double-D" shape so that the transmitter's own pulsing magnetic field cancels itself out across the receiver's pickup wire. Under normal conditions, with only air or clean dirt beneath the coil, the receiver hears absolute silence. Zero voltage. Total quiet.

Until a conductor walks into the beam.

The Ghost in the Metal: How Eddy Currents Fight Back

When that alternating magnetic field sweeps across a metallic object buried in the mud, something remarkable happens inside the atoms of that object.

Metals are full of free electrons that wander between atomic nuclei. When a pulsing magnetic wave crashes through them, it pushes those electrons around. Because the magnetic field is alternating, the electrons cannot travel in a straight line. Instead, they begin swirling in tight, circular paths right inside the metal itself.

Physicists call these eddy currents (opens in a new tab).

Now, here is the pivot that makes detection possible. Those swirling eddy currents do not just sit there quietly. Any electrical current moving in a loop generates its own brand-new magnetic field. And according to Lenz's law (opens in a new tab), that secondary magnetic field must push back directly against the external field that created it.

The buried coin has effectively become a temporary, battery-less transmitter. It radiates a faint electromagnetic echo upward through the soil.

As that secondary field radiates outward, it cuts through the detector's silent inner receiver coil. That sudden disturbance destroys the delicate induction balance. Electrons start fluttering through the receiver wire, creating a microscopic alternating voltage. The control box amplifies that signal by thousands of times, routes it through an audio processor, and converts it into a sharp beep in your headphones.

We rely on this identical electron-swirling trick every day at traffic intersections—the exact same induction principles power how traffic lights detect cars, where an embedded street loop spots the metallic chassis of your sedan.

The Phase Shift Secret: Telling Gold from Trash

If every piece of metal generates eddy currents, how does a modern detector avoid beeping at every rusty nail or bottle cap?

It measures time. More specifically, phase angle.

When the detector's transmitter coil emits a sine wave, that wave has a specific timing—peaks and valleys rising and falling at a fixed frequency. But the secondary wave returning from the buried target does not arrive back in perfect sync. There is a tiny, microsecond delay between the push of the transmitter and the kickback of the coin.

Engineers call that delay a phase shift.

What dictates the size of that delay? The target's ratio of electrical conductivity to magnetic inductance.

Take an iron nail. Iron is ferromagnetic, meaning it loves magnetic field lines and concentrates them easily, but rusty iron conducts electricity rather poorly. It reacts almost instantly to the detector's field without producing large, lingering eddy currents. The returning wave shows practically no phase lag.

Silver and copper sit at the polar opposite end. They are electrical superhighways. When the detector's field hits a silver coin, massive eddy currents spin up, creating substantial inductive inertia. The current takes time to build and time to collapse. That physical drag delays the secondary magnetic field, forcing the returning wave to lag well behind the transmitter's pulse.

Gold and thin aluminum sit right in the middle.

This is why your detector screams at a crumpled aluminum candy wrapper while barely whispering at a stainless steel butter knife—the crumpled foil spreads its conductive surface wide, creating an intense, shallow swirl of electrons that mimics high-value coin targets.

When the Ground Fights Back: Pulse Induction vs. VLF

In a laboratory, phase shift analysis works flawlessly. In the real world, the earth is messy.

Soil is rarely just neutral dirt. In many parts of the world, dirt contains red iron oxides, magnetite, and mineralized volcanic dust. Even worse is a wet ocean beach. Saltwater is a rich electrolyte loaded with dissolved sodium and chlorine ions. When a VLF coil sweeps across wet ocean sand, the salt water conducts electricity like a giant, continuous sheet of metal.

To a sensitive continuous-wave detector, the beach itself looks like one enormous, buried brass plate. The machine goes haywire.

This is why prospectors chasing gold nuggets in mineral-heavy Australian outback soil or divers combing shipwrecks turn to a completely different architecture: pulse induction (opens in a new tab) (PI).

A pulse induction detector throws out continuous sine waves altogether. Instead, it fires short, ferocious bursts of direct current—often over 1,000 high-voltage pulses every second—through a single coil.

Each pulse slams a strong magnetic field into the ground. Then, the detector abruptly cuts the power.

When that current shuts down, the magnetic field violently collapses, producing a sharp electrical spike (a phenomenon called back-EMF). If there is nothing metallic in the dirt, that voltage spike drops back to absolute zero almost instantaneously.

But if a gold nugget or brass cannonball is resting in the sand, the eddy currents trapped inside that metal cannot vanish in an instant. They linger. As those trapped currents slowly drain away, they generate a lingering magnetic tail that delays the voltage collapse by a fraction of a millisecond.

The detector listens to that decay curve. Because wet sand and loose ground minerals dissipate their electrical charge far faster than a dense piece of solid metal, a pulse induction unit can simply wait a few microseconds before taking its reading, effectively rendering mineralized ground completely invisible.

The walk-through metal detectors you pass at security checkpoints—operating alongside the advanced millimeter-wave systems explained in how airport scanners see through clothes—use similar pulsed electromagnetic arrays tuned to spot dense, hazardous bulk masses without getting confused by harmless pocket clutter.

(And honestly, the first time I watched an oscilloscope trace show a gold nugget holding an eddy current while black sand decayed to zero, I stared at the display for ten straight minutes.)

The Inverse Sixth Power: The Brutal Math of Depth

If metal detectors use such powerful physics, why can't hobbyists just turn up the dial and scan five meters deep into ancient Roman ruins?

Because geometry is brutal.

A search coil produces a dipole magnetic field. When that field travels outward from the transmitter, its field strength falls off with the cube of the distance ($1/r^3$). If you double the distance from the coil to the coin, the magnetic field reaching that coin is not cut in half—it is divided by eight.

$\text{Field at target} \propto \frac{1}{r^3}$

Then comes the return trip.

The tiny secondary magnetic field produced by the coin's eddy currents must travel all the way back up to the receiver coil. And that returning field also falls off with the cube of the distance ($1/r^3$).

When you multiply the two journeys together, the received signal strength plummets with the inverse sixth power of distance:

$\text{Signal received} \propto \frac{1}{r^6}$

That math is unforgiving. If a detector picks up a coin at 4 inches below the coil with a signal strength of 1,000 units, moving that exact same coin down to 8 inches (doubling the depth) slashes the signal by a factor of 64 ($2^6$). Push it down to 12 inches (tripling the original depth), and the signal collapses by a factor of 729.

This explains why even a $10,000 industrial detector rarely finds an isolated coin deeper than 12 to 18 inches. The physics simply runs out of juice. To push twice as deep, you need either an exponentially larger coil or vastly more wattage than a handheld battery pack can deliver.

Tuning Into an Invisible Chorus

It is easy to look at someone swinging a metal detector on a Sunday morning and see a quirky hobby. But underneath that simple plastic stem lies an astonishingly elegant dance of Maxwell's equations.

The machine does not need the target to be magnetic. It does not even need to touch the ground. It simply fires an invisible ripple into the earth, shakes loose the electrons sleeping inside ancient copper or forgotten gold, and measures the microsecond echo of their return.