How Does a Metal Detector Work?
At some point or another, most people have encountered a metal detector. Even if you are not a metal detecting enthusiast, it is more than likely that you have had a portable one waved over your person. If you are curious about how things work, you may have wondered just what makes a metal detector tick. Well, this article will work to shed some light on the internal workings of a metal detector.
Who Created the Metal Detector?
Well, as with most modern inventions, the actual metal detector cannot really be attributed to any one inventor. In 1881, Alexander Graham Bell did fashion a device that was an electromagnetic, metal locating machine. This design, however, was based off something that was invented by physicist Heinrich Wilhelm Dove.
Sometime later, an engineer Gerhard Fischer, filed a patent regarding a design. He then went on to build Fisher Research Labs which remains a prominent part of the metal detecting industry. However, it is probably Dr. Charles Garrett who was truly influential in producing the modern metal detector. He went on to patent his idea in 1987.
The Structure of a Metal Detector
Typically, a metal detector consists of a stabiliser, control box, shaft, and search coil. The stabiliser is not always present on every metal detector. Its sole purpose is to provide stability to the person who is handling the metal detector. The control box is the region that contains all of the circuits. It is with the control box, that the metal detector is both controlled and fine-tuned for specific conditions. The shaft is the attachment that allows the coil to connect with the control box. It also affords a certain amount of length adjustments to be made with the detector. The search coil can be thought of as the antenna – the part of the metal detector that is actually involved in the detecting.
How Metal Detectors Function
As it has been established, it is the coil that is actually responsible for the detection of metal. In fact, there are two different coils within the metal detector that are causing this detection. The outer coil is the transmitter coil while the inner coil is the receiver coil. This works to detect and amplify frequencies. This type of technology is known as Very Low Frequency or VLF technology.
When electricity is provided to this transmitter coil, there is a magnetic field created around the coil. As you move the coil, over the ground, you are causing the magnetic field to shift as well. Imagine, for instance, that you moved this field over the area of a metal object. This causes the electrons with the metal object to move, causing the magnetic field of the metal item to change. In addition to this magnetic field within the metal object, there is also a small electric current. This means that the metal detector has caused some electrical activity within the buried object.
Essentially, what is taking place is that due to the magnetic field of the metal detector, another magnetic field is created around the metal item. When this second magnetic field is created and the receiver coil is moved through it, electricity is created. This is what causes that difference in tone, causing a noise to be made.
Of course, metal detectors do more than just detect metal; they are also able to distinguish between the types of metal that they are detecting. This is done with the help of VLF phase shifting. A phase shift can be defined as the difference in timing between the frequency of a transmitter coil and that of the object.
This phase shift is due to either inductance or resistance. Inductance is something that very readily conducts electricity. As such, it takes a longer time to react to the changes in the current. Resistance, on the other hand, is an item that is less able to conduct electricity. Therefore, it reacts to the changes in the current much faster. This boils down to an object with high inductance producing a larger phase shift. For one with high resistance, however, the effect will be the opposite.
For objects made of gold, silver, copper, lead, nickel, or zinc, there is usually a large amount of conductivity involved. This means that they produced a larger phase shift. Iron, foil, and tin, on the other hand, have high resistance. The phase shift created is much smaller by comparison. Thus, this is how the metal detector discriminates between the different types of metal that are buried.
As you are aware, there are many different types of metal detectors available. Depending on the category that the detector falls into, there is typically a different type of technology being used. However, most consumer metal detectors use the structure and technology described in this article.