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How Metal Detectors Find Hidden Metal
Electromagnetic induction and eddy currents
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A metal detector finds hidden metal by using electricity and magnetism together. In the search coil, changing electric current creates a changing magnetic field that spreads into the ground. If that field reaches a buried coin, nail, or ring, it can make electric currents flow inside the metal. The detector senses the response and turns it into a sound, number, or display signal.
The key process is electromagnetic induction. A changing magnetic field induces swirling eddy currents in a conductor, and those eddy currents create their own magnetic field. The detector compares the field it sends out with the field that comes back, which helps it estimate size, depth, and type of metal. Very low frequency detectors use continuous waves, while pulse induction detectors send short bursts and listen for the fading return signal.
Key Facts
- A current in the search coil produces a magnetic field around the coil.
- A changing magnetic field can induce current in metal: induced voltage is related to ΔB/Δt.
- Eddy currents are circular currents that flow inside a metal object when it is exposed to a changing magnetic field.
- The induced eddy currents create a secondary magnetic field that the detector can sense.
- Faraday's law: induced emf = -N ΔΦ/Δt, where Φ is magnetic flux and N is the number of coil turns.
- Signal strength generally decreases as depth increases because the magnetic field spreads out and weakens with distance.
Vocabulary
- Search coil
- The circular coil in a metal detector that sends magnetic fields into the ground and senses returning signals.
- Electromagnetic induction
- The process in which a changing magnetic field produces an electric voltage or current in a conductor.
- Eddy current
- A looping electric current induced inside a metal object by a changing magnetic field.
- Magnetic field
- A region around a magnet or electric current where magnetic forces can act.
- Pulse induction
- A metal detecting method that sends short pulses of current through a coil and measures the delayed response from metal objects.
Common Mistakes to Avoid
- Thinking the detector sees metal directly is wrong because it senses changes in electromagnetic fields, not the object itself.
- Assuming only magnets can be detected is wrong because nonmagnetic conductors like copper, silver, and gold can still produce eddy currents.
- Moving the coil too high above the ground is a mistake because the magnetic field weakens with distance and the return signal becomes harder to detect.
- Treating every beep as the same type of metal is wrong because object size, shape, depth, soil minerals, and metal type all affect the signal.
Practice Questions
- 1 A detector coil has 80 turns. The magnetic flux through each turn changes from 0.003 Wb to 0.001 Wb in 0.020 s. What is the magnitude of the induced emf using emf = N ΔΦ/Δt?
- 2 A buried coin gives a return signal of 60 units at a depth of 5 cm. If a simplified model says signal is proportional to 1/d^2, what signal would you expect at 10 cm depth?
- 3 A gold ring and an iron nail are buried at the same depth. Explain why both can be detected, and why the detector might give different signals for them.