How do metal detectors work?

Why do you want? Want to detect metal? Oh, I don’t know… Maybe you want to find some gold in the ground. You can extract all the dirt, or you can find the location that contains the gold before You dig. Or maybe you’re looking for buried metallic meteorites. You can also use a metal detector to find the ring you lost at the beach. These devices are very useful.

But do you know how they work? Aha! When you think about it, it’s not obvious. There are different types of detectors, but they all rely on the same fascinating 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? Any solid body is made up of atoms, each of which contains negatively charged electrons orbiting a positive nucleus. In nonmetals such as plastic or glass, electrons are largely stuck to their parent atoms.

In a metal such as copper, the outer electrons swim freely and are shared by all atoms. That’s why electricity can flow through metal. If you apply an electric field, you get electrons flowing in a certain direction, which is what we call electric current. Metals are conductive.

Faraday’s law

So how do you create an electric field? The simplest way is to just apply a charge to the surface of a metal object by adding some electrons to it, and that’s what a battery does. Obviously this will not work for our purposes. You’ll need to get to the metal before you find it, which doesn’t make 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, the movement will create a changing magnetic field that produces an electric field. If this electric field were present in a metal spike: you would get what is called an eddy current.

vice versa

It goes the other way too: just as a changing magnetic field creates an electric current, an electric current creates a magnetic field. Remember the old science fair project where you wrapped a wire around an iron nail and connected the ends to a battery? As the juice flows, the nail temporarily becomes magnetic and can pick up paper clips.

But as we just saw, you don’t need a battery. The changing magnetic field creates eddy currents in the metal, and these eddy currents then form their own magnetic fields. I am waiting! It’s even crazier. Since these eddy currents create magnetic fields, there will be an interaction between the metal and the object that creates a changing magnetic field.

Now you’re ready for your first very simple metal detector. To create a changing magnetic field, we will just use a moving magnet. In the demo below, I placed a magnet over the coin and then quickly pulled it out. The movement creates eddy currents in the coin, and these currents form a magnetic field that interacts with the magnet. See? Coins jump.