How do metal detectors work?

why you Want to detect metal? Oh, I don’t know… maybe you want to find some gold in the ground. You can dig all the dirt, or you can find a place that has gold before You dig, or maybe you’re looking for buried metal meteorites. You can even use a metal detector to find a ring you lost on the beach. These devices are quite useful.

But do you know how they work? Aha! When you think about it, it’s not obvious. There are many different types of detectors, but they all use the same cool physics of electric and magnetic fields. Let’s take a look, shall we?

go with the flow

First, what distinguishes metals from other materials? Every solid body is made up of atoms, each with negatively charged electrons buzzing around a positive nucleus. In non-metals such as plastic or glass, the electrons are attached almost exclusively to their parent atom.

But in a metal like copper, the outer electrons swim around freely and are shared between all the atoms. This is why electricity can pass through a metal—if you apply an electric field, you get a flow of electrons in a certain direction, which we call an electric current. Metals are conductors.

Faraday’s law

So how do you create an electric field? The simplest way is to apply a charge to the surface of a metal object by just adding some electrons to the surface of the metal object – that’s what a battery does. However, it obviously won’t work for our purposes. Before you can find the metal, you have to access it, which makes no sense.

But there is another way to go. It turns out that a Changing the magnetic field It also creates an electric field. This is the basic idea of ​​Faraday’s law. If you move a magnet near a metal conductor, this movement creates a changing magnetic field, which creates an electric field. If that electric field is in a metal – boom: you get something called 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 end to a battery? When the juice flows, the nail becomes temporarily magnetized and can grab the paper clips.

But as we’ve seen, you don’t need a battery. A changing magnetic field creates eddy currents in a metal, and these eddy currents create their own magnetic fields. Wait this is crazier. Because these eddy currents create magnetic fields, there will be an interaction between a metal and something that creates a changing magnetic field.

Now you are ready for your first very simple metal detector. To create a changing magnetic field, we just use a moving magnet. In the demo below, I placed a magnet on a coin and then quickly pulled it up. This movement creates eddy currents in the coin and these currents create a magnetic field that interacts with the magnet. see Coins jump up.