How Do Metal Detectors Work?
Why would you want do you want to detect metal? Oh, I don’t know…maybe you want to find gold in the ground. You can dig up ALL the dirt, or you can find the place that contains the gold. Before you dig. Or maybe you’re looking for buried metallic meteorites. You can even use a metal detector to find that ring you lost at the beach. These devices are very useful.
But do you know how they work? Ah! When you think about it, it’s not easy. There are different types of detectors, but they all rely on the same physics of electric and magnetic fields. Let’s take a look, shall we?
Go with the flow
First, what makes metals different from other materials? Every solid object is made up of atoms, each with negatively charged electrons buzzing around a positive nucleus. In non-metals like plastic or glass, electrons practically stick to their original atom.
However, in a metal like copper, the outer electrons swim freely and are shared by all atoms. This is why electricity can flow through a metal: if you apply an electric field, you get a flow of electrons in a certain direction, which we call electric current. Metals are conductors.
Faraday’s law
So how do you create an electric field? The simplest way is to simply apply a charge to the surface of a metal object by adding a few electrons: this is what a battery does. Obviously this won’t work for our needs. You would need access to the metal before finding it, which makes no sense.
But there is another way forward. It turns out that a change of magnetic field also creates an electric field. This is the basic idea of Faraday’s law. If you move a magnet near a metal conductor, the movement will create a changing magnetic field which produces an electric field. If that electric field is in a metal, boom: you get what’s called an eddy current.
And vice versa
It also goes the other way: just as a changing magnetic field creates an electric current, an electric current creates a magnetic field. Remember that old science fair project where you wrapped a wire around an iron nail and connected the ends to a battery? As the juice drains, the nail temporarily becomes magnetic and can pick up paperclips.
But as we just saw, you don’t need a battery. A changing magnetic field creates eddy currents in a metal, and these eddy currents then create their own magnetic fields. Wait! It’s even crazier. Because these eddy currents create magnetic fields, there will be an interaction between a metal and the object creating a changing magnetic field.
You are now ready for your first very simple metal detector. To create a changing magnetic field, we will simply use a moving magnet. In the demo below, I placed a magnet on a coin and then quickly pulled it. The movement creates eddy currents in the room, and these currents create a magnetic field that interacts with the magnet. See? The pieces jump.
