How Does a Metal Detector Work?

The flat disc at the end of the shaft is really two coils of wire. One transmits: electricity runs through it and generates a magnetic field that pushes down into the soil. The other receives: it listens for magnetic fields coming back up.

When that transmitted field passes through metal, it induces small swirling electrical currents in the object — eddy currents. Those currents generate their own weak magnetic field, and that's what the receiving coil hears. No metal, no echo, no beep.

How strongly a target echoes depends on its conductivity — how easily electricity moves through it — and on its size and shape. Silver conducts extremely well, iron poorly, gold sits awkwardly in between.

The detector measures the strength of the returning signal and how much it lags behind the transmitted one, then maps that onto a number: the target ID. Iron reads low, foil and gold read low-to-middle, coins read high. This is why an experienced detectorist can often call a target before digging.

Discrimination is the detector ignoring targets whose ID falls in a range you've rejected — usually iron and bottle caps. Set it too high and the machine goes usefully quiet; set it much higher and you start rejecting good targets that happen to read nearby.

This is why experienced hunters often run less discrimination than beginners. A gold ring and a pull tab can read almost identically, so filtering out all the junk also filters out the ring.

Soil isn't neutral. Iron-rich, mineralized ground responds to the detector's field, producing a constant background signal that masks real targets. Ground balance is the detector measuring that background and subtracting it.

Cheap machines have it preset at the factory, which works in average soil and fails in bad soil. Better machines let you set it automatically or by hand — and in heavily mineralized ground, that adjustment matters more than any depth spec on the box.

Frequency is how many times per second the coil's field pulses, measured in kilohertz. Low frequencies (around 5 kHz) travel deeper and favor big, highly conductive targets like silver coins. High frequencies (40 kHz and up) are more sensitive to small, low-conductivity targets like fine gold.

A single-frequency machine forces a choice. Multi-frequency detectors transmit several at once and compare the returns, which is why they can find small gold and deep silver in the same swing — and why they can separate the salt signal on a wet beach from actual targets.

Most detectors are VLF (very low frequency) — the two-coil transmit-and-receive design described above. VLF machines can identify targets, which is why they have target IDs and discrimination.

Pulse induction machines work differently: they send strong pulses and time how long the echo takes to fade. They go deeper and completely ignore ground mineralization, which makes them the choice for saltwater and heavily mineralized goldfields. The trade-off is that they can barely discriminate — you dig everything.