# How does a metal detector work ?

1. Oct 30, 2010

### Fairytales

Hey there,

I have problems trying to understand how a metal detector works. I don't understand why the inductance of a coil changes when a metal is approaching. Can someone explain that to me ?

Thanks a lot for your help

2. Oct 30, 2010

### Theaumasch

There's a coil inside a metal detector which emits EM-waves. If there's a metal nearby, the EM-waves induce currents inside that metal. Because currents are a source of magnetism, an EM wave will also be emitted by the metal. Both waves are detected by a second coil inside the metal detector.

If there's no metal near the detector, the measured field is just the detector's field. If there's a metal nearby, the extra magnetic field will be measured.

If something's still not clear, please say so. I mainly just copied the article on this subject on wikipedia.

3. Oct 31, 2010

### cragar

When you say EM-waves i think of photons , The coil inside the metal detector creates a changing magnetic field , and if this B field is near another conductor it will cause electrons to flow in the material and create its own B field that will oppose the one in the detector "lenz's law". Just like dropping a magnet down a copper pipe .

4. Oct 31, 2010

### dreit

For those extra high-tech metal detectors that tell you how far down something is, does that mean that the actual detector just measures how far down it sends those EM signals?

5. Oct 31, 2010

### schip666!

The <u>way</u> basic metal detector is just a coil of wire with an air core. The coil has a specific inductance and is used as a resonant component in an LC oscillator, so, when sitting all alone by itself, the oscillator has some specific frequency. When a piece of metal (especially a magnetic metal like iron) is brought near the loop it's inductance changes -- the how of this is beyond my paltry grasp of basic physics, sorry, maybe see the above quoted Lenz's Law -- and thus the oscillator's frequency changes. The rest of the exercise is in detecting the frequency change.

For higher sensitivity one can use multiple loops and all the rest of modern electronics as described in the other posts here...