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.)
