Why would you want to detect metal? Oh, I don’t know … maybe you want to find some gold in the ground. You could dig up ALL the dirt, or you could find the location that has the gold before you dig. Or maybe you’re looking for buried metallic meteorites. You could even use a metal detector to find that ring you lost at 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 different types of detectors, but they all draw on the same cool physics of electric and magnetic fields. Let’s take a look, shall we?
Go With the Flow
First, what makes metals different than other materials? Any solid object is made of atoms, each with negatively charged electrons buzzing around a positive nucleus. In nonmetals like plastic or glass, the electrons pretty much stick to their original atom.
In a metal like copper, however, the outer electrons swim around freely and are shared by all the atoms. That’s 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 electrical current. Metals are conductive.
Faraday’s Law
So how do you make an electric field? The simplest way is to just apply a charge on the surface of a metal object by adding some electrons to it—this is what a battery does. Obviously that won’t work for our purposes, though. You’d need access to the metal before you find it, which makes no sense.
But there’s another way to go. It turns out that a changing magnetic field also makes an electric field. This is the basic idea of Faraday’s law. If you move a magnet near a metal conductor, the motion will create a changing magnetic field that 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 goes the other way too: 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 wrap a wire around an iron nail and connect the ends to a battery? When the juice flows, the nail becomes temporarily magnetic and can pick up paper clips.
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 make 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 thing making a changing magnetic field.
You are now ready for your first, very simple metal detector. To make a changing magnetic field we’re just going to use a moving magnet. In the demo below, I put a magnet on top of a coin and then pulled up quickly. The movement creates eddy currents in the coin, and these currents make a magnetic field that interacts with the magnet. See? The coins jump up.
To be clear, when the magnets are in contact with the quarters and stationary, there is no attraction at all. These quarters contain no magnetic metals. But when I move the magnets away, it creates eddy currents in the coins that make them temporarily magnetic.
Why does the coin in front show a stronger effect? It’s a 1959 quarter that’s mostly made of silver. The other is a modern quarter made of a copper alloy. Because silver has a lower electrical resistance than copper, the changing magnetic field makes a stronger eddy current in that one. Nice! It’s not just a metal detector, but one that can tell the difference between copper and silver.
Of course, walking around with a magnet on a stick, bobbing it up and down, would be a tiresome and impractical way of looking for metal. We need a better method.
Look Ma, No Magnets
In fact, we don’t even need magnets. In the next demo I have a coil of wire wrapped around an iron core. This is a four-step process: (1) When the coil is plugged into an outlet, we get an alternating electric current. (2) This changing current creates a changing magnetic field in the iron core. (3) The changing magnetic field induces an electric current in the aluminum ring. (4) This induced current makes a secondary magnetic field, but with an opposing polarity, so it creates a repulsive force. Voilà, the ring is launched into the air! How cool is that?
Got it? We went from a changing electric current in the coil to a changing magnetic field in the iron core to an electric current in the ring to a magnetic field in the ring. Wouldn’t it be great if you could just make buried gold jump out of the ground? Sadly, this isn’t practical either, but it points us toward a solution.
Real Metal Detectors
In fact, most metal detectors use this same idea. Basically, you’re shifting an electric current from your device to a piece of metal underground and back to your device, causing it to beep. (And remember, this works because only metals conduct electricity.) The means of shifting the current is the creation of magnetic fields. This is how wireless chargers work too!
So how do we detect that telltale magnetic field from an underground object? One way is to add a second coil in the device, so we have an emitter coil and a receiver coil. The only problem is that the first coil will already induce a current in the second coil. But there are some tricks we can use to eliminate the interference.
For example, if we position them just right so that their opposing magnetic fields overlap and negate each other, we can have a net zero magnetic effect. We call these “balanced coils.” Then, when a piece of metal is nearby, the added magnetic field will make them unbalanced, and there’s your detection.
That Resonates With Me
Another fun trick is to detect metal using resonance. If you’ve ever pushed a kid on a swing, you know about resonance. The swing moves back and forth with some natural frequency (determined by the length of the chains). If you push the swing with the same frequency, even with just a light touch, it will go higher and higher. Push at the wrong time and you mess it all up.
In general, with any oscillating object, applying an external force at the same frequency increases the amplitude of the oscillation. That’s resonance. This is what happens when a singer shatters a wine glass by holding a note at a pitch that matches the particular vibration frequency of the glass. (It’s the pitch control, not the loudness of opera singers that lets them do this.)
We can harness resonance as a signal by creating an oscillating circuit. To do this, we hook up an inductor (our coil of wire) and a capacitor, which is a component that stores energy in an electric field. Now, if you hold this system over a buried piece of metal, it will change the oscillation frequency of the circuit. Ping! Treasure detected!
What you’re hoping for, of course, is something like the 13th-century gold coin found by a guy in England, which sold for $850,000. However, if you’ve ever actually used a metal detector, you know that a lot times it’s a pull tab from a soda can. That’s OK. The fun is in the hunt—and figuring out why it works.
