We have all heard of metal detectors; they are everywhere even though you may not know it. If you have been on holiday, you have walked through one at Airport security, and some of us may even have had the luxury of playing with a handheld one. In practice, they work wonders; for example, stopping terrorists from travelling around or even when looking for your parent's ring in the garden after a family barbecue. How they work is fascinating and this is what this article is about!

Electricity and Magnetism

When people see these two words they tend to run away - but please do stay! This will make sense and it certainly is interesting, I promise!

What is their relevance?

Imagine an inseparable pair of best friends or a madly in love married couple; this is what electricity and magnetism are like. Where you see an electric field you see a magnetic field. Without this cute relationship between them, there are a lot of things we stop being able to do; for example, using a washing machine, vacuum cleaning, playing with computers - in fact, generating electricity would be impossible.

Electromagnets

Electromagnets are simply the combination of electric fields and magnetic fields in some sort of machine. Let me tell you how electricity is generated using them, by talking about wind turbines (Figure 1) as an example. When wind blows the turbine's arms around, this motion causes copper wires inside it to start moving around. These wires are kept between a couple of magnets. What you now need to know now, is this golden rule:

When a conducting object (like a metal) is moved around a magnetic field, a current is generated in the metal. If there is a current in a metal there is a magnetic field around this current.

Mouthful, yes, but it will all make sense in due course. This is why wind turbines are a renewable source of energy because you cannot run out of wind and nothing is being wasted making this electricity.

Figure 1: Image showing wind turbines

So how do metal detectors work?

The metal detector is basically made of 2 coils in separate circuits. One is called the transmitter coil, the other is called the receiver coil. When you switch the detector on the current flowing through the transmitter coil 'creates' (or induces) a magnetic field. When you start to wave your detector about you are moving this magnetic field.

Remember our golden rule?

If this magnetic field passes through a metal ring, which conducts electricity, what happens to it? A current is produced. Now since there is a current inside this ring, again, using the golden rule, it induces a secondary magnetic field around it (the ring). This secondary magnetic field is picked up by the receiver coil in the detector, which is connected to a buzzer.

Again, referring to our golden rule:

The magnetic field in the metal ring causes a flow of current in the receiver coil which causes the buzzer to go off. There you have it, how a metal detector works!

Hope this did end up interesting you and if you have any questions, leave them below!

Until next time, take care. Mystifact

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