How Metal Detectors Work
From protecting soldiers on the field of battle to entertaining children on the beach for hours, the metal detector is one of society's most versatile inventions. Is it a toy, or is it a tool? That's up to you to decide. The science that is responsible for creating the magic - now that's something not up for debate.
Let's take a closer look at how the metal detector works.
What is the Science Behind the Metal?
The phenomena of electromagnetism is responsible for the power of the metal detector. Electromagnetism is a fundamental part of the world that we live in - the marriage of two of the most incredible forces in that world (electricity & magnetism).
The metal detector creates a current of electrons and pushes them along a conductor. Whenever and wherever this happens in the universe, a magnetic field pushes out from this current. It flows in a direction that is perpendicular (90 degrees) to the current. The opposite also works as well. In a conductor, if there is a conducive material that flows across a magnetic field, you can be sure that a perpendicular current will be induced.
How does the metal detector work? It simulates the first of the above situations, creating a current of electrons that flow through the transmission coil in the head of the tool. Cue physics - the magnetic field that runs perpendicular to that current will undoubtedly show itself. It extends away from the coil and surrounds the head, getting stronger or weaker based on the strength of the current and the speed of the conductor's winding.
If any sort of metal comes into contact with the magnetic field that is created in the detector, it will come into contact with the electron current. This induction creates a phenomenon that is known as "eddie currents" inside of the metal, which simulates a circular tide. In response, the currents begin to create a weak magnetic field that runs against the original magnetic field that was created before.
The search coils that are in the detector, also known as the detector's antenna, have the ability to pick up the presence of this weak magnetic field. Once metal is detected, another mechanism simply alerts the user that a new piece of metal is coming into the original field. The unique signature of metal that causes the alarm comes from a characteristics that is known as resistivity. Metals have a resistivity that is unique to them and much different from other materials. This is why you don't get an alarm when you run into a rock or a seashell while metal detecting.
Modern metal detectors actually have the ability to distinguish between the different types of resistivity that are in different metals. Although there are some limits to this ability, the best new detectors can get relatively precise in letting you know what kind of metal you are running into. This can be a huge help if you are looking for an expensive silver earring in a room that is full of old pennies.
A Short History of the Metal Detector
The metal detector first came into mainstream use after the July 1881 shooting of American president James Garfield. One of the bullets that hit President Garfield was buried so far inside of his body that doctors could not find it. Lewis Lattimer, draftsman and assistant to Alexander Graham Bell, led Bell in quickly putting together a functional metal detector that they called an "induction balance." The device did successfully find the bullet, although it was too late to actually save the life of the President. The induction balance thus became the first metal locator to work from the concept of electromagnetism.
The next big breakthrough in electromagnetic metal finders came circa 1933. Gerhard Fischer, a German electronics engineer living in the United States, applied for a patent on his portable metal detector. He referred to his creation as the "metalloscope." As he did this, Fischer also founded his own research lab, which today is one of the top manufacturers of metal detectors in the entire world.
In the 1970s, Dr. Charles L. Garrett brought what we know as the modern metal detector to the commercial market. The device was created through his business Garrett Electronics. Garrett, like many people who use the metal detector today, had a serious interest in amateur treasure hunting. He also had the chops to seriously improve on the past iterations of the metal detector, as he had previously worked on the NASA moon landing program. As it turns out, Garrett was the man who revolutionized the metal detector through a number of insightful innovations. Among these is the very first metal detector that made use of a computer and featured digital signal processing. This idea was patented in 1987.
How Do I Use the Science of the Metal Detector in My Favor?
Now that you know the basics of metal detection, you may think that you are ready to go out and find your buried treasure immediately. Not so fast! There are some nuances about electricity and magnetism that you should know. They will affect how you work your metal detector and the luck you have out on the field.
There is a reason that metal detection is viewed as an art and as a competitive sport in some places. Understanding the science behind the detector means that you master this science more thoroughly. Let's take a look at some of the details of electromagnetism and metal detection that makes it an art form as well as a science.
1. Why do you need to keep your metal detector moving in order to find the good stuff?
There is a scientific reason that you always need to keep your metal detector moving in order to make a big score. If you stop it, even if you are inches away from a huge chunk of metal, you may not be able to detect it. If the detector stops moving, then the magnetic field that you are creating with the battery that activates the transmitter may not penetrate the metal that is in the ground. If the field is not created, then you never get the electric current that creates the second magnetic field that sets off the alarm.
2. How deep can a metal detector actually detect metal?
This depends on many different factors. Of course larger objects are easier to find than smaller ones, especially at lower depths. However, objects that are buried in a flat way are easier to find than objects that are facing downward. There is a larger surface area to connect with the detector if the object is flat, but this is not the only factor here. Flat objects are also more likely to send their magnetic fields back to the detector and setting off your alarm. In most cases, you can count on a metal detector to work to a max depth of 50 cm.
3. Are older objects more difficult to find than newer ones?
You might think that the age of metal does not matter, but it does. Yes, metal does disintegrate at a much slower rate than other types of objects. You will probably hot be able to tell the difference between a penny that has been left in the ground for two days versus two years. However, there are other things that go on that the naked eye cannot see. Oxidation and corrosion are not as visible, but they both happen faster than visual disintegration. Metals corrode more quickly than you may think. The longer that an object is in the ground, the tougher it will become for you to find.
4. Does the surrounding soil make a difference?
The density of the soil that is surrounding the object you want to find makes a difference! Basically, the denser the soil, the more that your object hides.
Where Can Metal Detectors be Used?
The stereotype of the metal detector is some sunburned guy or naive kid on a beach, blindly looking for random coins. This is far from the only use of the metal detector in the real world. Because of the multi frequency potential of metal detection, including the ability to set them to a very low frequency pulse induction, the metal detector can be used in airports, in prisons and in hospitals. Metal detectors are also used in many different kinds of scientific research.
Depending on how they are used, detectors can be disruptive to a particular environment or quite helpful. For instance, the metal detectors that are in airports are set to specifically catch the metal that is usually contained in weapons like knives and guns. However, setting the same detector to the same frequency can cause a great deal of disturbance to an archeological site, for instance. This is why archeologists often disallow untrained people from using metal detectors on the sites of important artifacts.
On the opposite end of the spectrum, a properly calibrated metal detector has helped to unearth many important pieces of metal from the past.
Can My Metal Detector Actually See Through the Ground?
Well, the answer here is yes and no! Strictly speaking, the metal detector that you are using cannot see much of anything other than magnetic fields that cut off its own magnetic field. Because of this, the basic metal detector can be fooled into thinking something is metal when it is not. The industry has fixed many of these problems, but some of the solutions make use of science that is outside the realm of electromagnetism and this article.
In order to understand whether your metal detector can actually see through the ground, you might want to imagine your own eyes and how they view things. Imagine you could only see the color blue. All other colors were impossible for you to see. You may be able to view the outlines of things in your vicinity, but you would not be able to view them with the same detail as you do now. With practice, you may be able to identify items. However, you would probably have to interact with those items using your other senses - smell and touch, most likely. You will only have a full representation of what an object is once you have built on the basic "technology" of your eyes.
The metal detector that beeps when it runs across a certain magnetic field is like the eyes that can only see blue. It does not actually know what it is seeing. It can be stopped from seeing it if the soil is too dense. However, you can look at a metal detector as seeing through the ground, because it is reacting in a way that identifies something with a clear set of characteristics. Were it tuned to other things besides metal, it would be able to "see" those things as well.
The metal detector is one of the most entertaining and popular applications of science ever invented. Hopefully, understanding some of the science, history and philosophy behind it can increase your enjoyment even more. Good luck finding your hidden treasure!