Metal Detector: How Does It Work? | Coil Physics Explained

Metal detectors work on physics that predates the space race, yet the principle remains unchanged in machines from budget hobby units to industrial sweepers. Understanding the mechanism — how a coil of wire creates a field, how that field excites metal, and how the detector hears the echo — tells you more about choosing and using a unit than any spec sheet. It also explains why some targets pop out of the ground easily while others stay stubbornly silent.

The Core Mechanism Behind Every Metal Detector

A metal detector’s heart is a search coil, usually a loop of copper wire housed in the rounded plastic disc at the end of the shaft. The control box sends an alternating current through this coil, which generates a changing magnetic field that extends into the ground below. When that field encounters a conductive metal object, it induces small circulating electric currents — eddy currents — inside the target. Those currents generate their own magnetic field, which opposes the original field.

The detector’s electronics sense this disturbance, typically through a second receive coil or a sampling circuit that measures voltage changes. The signal gets amplified and processed, then converted into the audio tone and display reading you see. The National Institute of Justice’s User’s Guide to Hand-Held and Walk-Through Metal Detectors documents how these systems operate and how operators interpret their signals.

This is the essence: transmit a field, induce currents in metal, detect the reaction. The entire industry — from $100 hobby machines to airport security portals — runs on this same loop.

VLF vs Pulse Induction: Two Ways To Listen

Two main coil technologies dominate consumer and professional metal detectors, and they take opposite approaches to the same task. Very Low Frequency (VLF) detectors use two separate coils — a transmit coil and a receive coil — wound concentrically. The transmit coil produces the search field; the receive coil sits in the null region of that field and only picks up the secondary signal coming back from a target. This arrangement gives VLF machines strong target discrimination, letting them distinguish between, say, a nickel and a pull tab.

Pulse induction (PI) detectors ditch the transmit/receive split entirely. Instead, a single coil fires short, high-current bursts of energy, then switches to listening mode between pulses. The coil measures how quickly the reflected magnetic field decays — metal targets slow that decay in characteristic ways. PI machines lose the fine discrimination of VLF units but gain raw depth and the ability to ignore mineralized ground that confuses VLF detectors. Beach hunters and gold prospectors favor PI for exactly this reason.

Why Some Targets Are Easy To Find And Others Vanish

Detection strength depends on four variables, and knowing them explains why your first trip out produces a pocketful of bottle caps and nothing else. Target size matters most — a larger object reflects a stronger signal. Composition matters too: highly conductive metals like copper, silver, and aluminum respond more strongly than low-conductivity iron. The target’s orientation relative to the coil changes how efficiently eddy currents form, which is why a coin lying flat gives a louder signal than the same coin standing on edge.

Distance rules everything. Signal strength falls off sharply as the target moves deeper, and each machine has a hard depth ceiling beyond which even a large object stays invisible. Sweep technique exists to manage these variables — a slow, steady pass keeps the coil at a consistent height and lets the detector integrate returning signals properly.

Choosing A Detector That Matches How The Tech Works

Once the mechanism makes sense, the buying decision simplifies dramatically. VLF detectors like the Garrett ACE 300 — currently priced at $269.99 on Garrett’s USA site — suit beginners and coin shooters because of their discrimination capabilities. A pulse induction machine costs more but earns its keep on saltwater beaches and in iron-heavy soils. If you are deciding between them for your first unit, our roundup of all-round metal detector picks walks through tested models for different budgets.

Whatever you choose, practice matters more than price. Learn how your specific machine voices its signals, test it against known targets in your own yard, and you will out-find someone with a machine three times the price who never learned its language.

That table condenses the trade-offs. A VLF machine tells you what you found before you dig; a PI machine shows you where it is, deeper. Neither perfectly does the other’s job, so match the tech to the ground you actually plan to hunt.