How do metal detectors work?

How do metal detectors work?

How do metal detectors work and how is this different from powering an entire city? Well, there really isn’t a difference per say as the same underlying fundamental physics applies to both.

Strictly speaking, exactly how do metal detectors work is rooted in a fundamental force of nature and in this article we will be taking a look at the science behind the metal detector.

It is harder to understand the electromagnetic field than to understand invisible angels; the former requires a vivid imagination of a myriad of complex waves
— Feynman, R.P., Leighton, R.B., Sands, M., The Feynman Lectures on Physics, 1964

Electromagnetic Force

To know the science of metal detecting one must first know the science of electromagnetic force. Electromagnetic force is a interrelated phenomena that occurs between electricity and magnetism. Although, in practice, these two distinct quantities behave quite differently, they are in reality the opposite sides of the same coin.

This principle is very important because it is extremely pervasive in our day to day lives. Electromagnetic force is responsible for things as simple as being able to hold an object in your hand (as opposed to it passing right through), to as phenomenal as levitating trains in the Asian and European transit system, to as powerful as supplying almost all the energy we use today.

Let’s investigate exactly what is going on here and how it best can be explain. We will start with a electrical generator and a electrical motor.

Electric Generator

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Electric Generator – manually rotating the coil across magnetic fields induces electrical current.

The electrical generator is a mechanism that manually (mechanical force) rotates a loop of conductive material (usually copper) through a magnetic field such as that produced by a permanent magnet.

The magnetic field has both a positive and negative side (north/south respectively) and the rotating conductor loop will move through (cut across) the positive and negative areas of the magnetic field. This induces electrons (negatively charged particles) to flow towards the positive side of the magnetic field.

This flowing of electrons in the conductor is called electromagnetic induction and the electrical force manifested in this process is usually routed through brushes on the shaft of the generator. The amazing part of this process is that, if scaled large enough, an entire city can be powered with this induced electricity.

Electric Motor

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Electric Motor – generating a current in a coil with in a magnetic field causes the coil to rotate.

The electrical motor is mechanism that reverses the processes of the generator. Instead of the conductive loop being rotated manually, an electrical current is passed through the loop. The electrical current or electron flow creates a surrounding positive and negative field about the loop.

The permanent magnet will then attract the oppositely charged field while repelling the like charge. This creates a rotation in the negatively charged (opposite the flow of the electrons)  direction of the magnet. This effectively creates mechanical energy from electrical energy and is the force basis of many electrical devices.

With the electrical generator, electrical motor and a power transport system, we are able to almost instantly from almost anywhere have access to unlimited mechanical power.

Electricity is really just organized lightning
— George Carlin

There is one last device I would like to introduce before we pursue these principles in metal detectors. Enter the electric magnet, a device that will shed a different but import light on the integral relationship between electricity and magnetism.

Electric Magnet

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home made electric magnet

The electric magnet can be made fairly simply with a nail, a length of insulated copper wire and a battery. The copper wire is wound comfortably around the nail and then the ends of the copper wire are connected to the negative and positive sides of the battery.

If you try this experiment at home you will be able to pick up paper clips and staples with the nail. You would also notice three things. First, the more battery power the more paper clips or staples you will be able to pick up because as the current increases the power of the magnetic field increases. Second, by applying more turns of the copper wire to the nail, the concentration of the magnetic field will intensify making the magnetic field stronger. Third, The nail will become magnetized. That is, the atoms in the nail will line up like soldiers in formation creating strong positive and negative poles.

What is important to note is that the poles of the nail are perpendicular to the coil of copper. To think of this another way, if the wound copper wire was a flat disk, it would have a positive side and a negative side.

Hopefully, given these examples, you are just dangerous enough to start understanding the science of metal detectors.

How do metal detectors work

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With the basics in hand, lets now move are attention to how do metal detectors work.

For this article we will focus on the common VLF (very low frequency) also call inductive balance variety of metal detectors using a standard concentric coil. In anatomy of a metal detector this kind of metal detector is portrayed.

A metal detector works like a electric motor. The power source (batteries) in the control center of the metal detector sends a current down the shaft and into the metal detectors outer coil called the transmitter coil. The coil windings in the transmitter coil in turn generate magnetic fields. The amount of current and the windings in the coil determine the strength of the magnetic field just like that of a electric battery. The magnetic field is generated perpendicular to the circulating current and if the coil is held parallel to the ground, the magnetic field will penetrate into its depths.

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Magnetic field lines around a coil circulating a electrical current

The control center is also responsible for pulsating the current in opposite directions. Each pulse of current will reverse the current’s direction and, thusly,  the polarity of the magnetic field (from north to south to south to north). This gives the magnetic field permeating the ground a push-pull effect energizing metals with in its field.

The pulsating magnetic field will create tiny electric generators in metal under its coil. From our discussion on electric generators we know that when a magnetic field cuts across a copper winding in motion, current is generated. The same principle applies here except that it is the magnetic field in motion both by its pulsation and by the handler of the metal detector moving the magnetic field.

The currents that are generated in this way are called eddie currents. Eddie currents in metals are a well known phenomena and are dependent upon a number of things, most notably the conductivity of the metal. The more conductive the metal the less resistive it will be and the better the current will flow.

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The many eddie currents generated by a magnetic field cutting across a conductive metal surface. Notice the magnetic field lines generated by the eddie currents.

With the generation of eddie curents, we come back to the principles of electromagnetism in that when there is a electrical current present there will be a magnetic field. In effect, we have created a tiny electrical magnet that will generate a magnetic field in opposition to the source magnetic field. This very weak magnetic field can be sensed by the inner coil also called the receiver coil of the metal detector. The inner coil is shielded against the outer coil (influence of the transmitted field are cancelled out) as to not effect it purpose.

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reflected magnetic fields

How do metal detectors work is simple matter when the concepts are understood. The metal detector transmitter coil generates a magnetic field that penetrates into the ground underneath. The metal caught with in this magnetic field generate eddie currents. Eddie currents in turn generate a magnetic field that is reflected back to the metal detectors receiver coil. The receiver coil detects these tiny fields and then processes, amplifies and alerts the user that metal has been found.

I hope that this article was helpful in your understanding of how do metal detectors work.

Until next time, Good Digging!

  1. Ariel
    • John Brunelle

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