Metal detectors are said to date all the way back to the year 1881 when a United States President called James A. Garfield. When President Garfield was shot they were unable to find the missing bullet, which is when telephone inventor Alexander Graham Bell quickly made an electromagnet metal finding device called an “induction balance”. This was loosely based on an earlier Invention by a German physicist called, Heinrich Wilhelm Dove. Unfortunately the bullet was never actually found and the president died. But the metal detector was working just fine but perhaps needed a few tweaks or the bullet may not have even been there.
Mobile metal detector where later invented by another German born gentleman called Gerhard Fischer. He was living in the United States at the time of his invention and eventually applied for the patent in January 1933. Gerhard decided to call his invention a:
Metalloscope– “a method and means for indicating the presence of buried metals such as ores, pipes and the like’
Fischer also founded the Fisher Research Laboratory which is still in business today of making metal detectors.
Others that followed were:
Early 1970’s – Dr Charles L Garrett founded Garrett Electronics – pioneered modern metal detectors. Garrett worked for NASA for while before his company revolutionised the metal detector by making it computerised.
What Are Metal Detectors Used For?
Metal detectors are not only used for beach combing. You will see them at airports as walk through scanners. Or on a night out you will see security doing checks with a portable scanner. These are used to detect weapons in both cases. In the case of the army they will use a sweeping metal detector to detect mines hidden in the ground. They can also be found being used in scientific research although Archeologists frown upon the use because it disturbs the natural lay of the earth.
What Are The Different Types Of Metal Detectors?
Metal detectors were mainly designed and developed for detecting explosives, Refinement of Alexander Bells original invention have lead to different types, but they all still work on the same concept of electricity and magnetism.
Best Frequency Oscillation – The oscillation detector is very easy to understand, and perhaps the most basic. Its depth of reading is around 2 Feet. All metal detectors have a ring of coil copper rolled around iron or steel. There is one large ring at the base of the detector and one smaller one a little higher up. The coils are both connected to oscillators that generate a frequency.
The coils operate at a different frequency to each other, which is what the speakers will detect. (Speakers can also be in the form of headphones). The audio will change from a steady hum to an interference with the frequency transmission when a metal object is hit and the electromagnetic field is interrupted.
Very Low Frequency Detectors – This detector is most favored buy the treasure hunters out there. It is an accurate sensitive device due to its coil, which has a pair of coils. The first coil is the transmitter with an intense electrical current. The second coil acts as the receiver collecting the signals and amplifying them.
The transmitter coil works by producing a magnetic field above and below the ground. Again as the best frequency oscillation this can reach about 2 feet under the ground. When a metal object is swept the object develops a magnetic current, interfering with the already stable magnetic loop running into the ground. The second coil picks up the interference, magnifies it and then sends it to the speakers or audio system used.
The magic on this detector is that it picks up a different frequency depending on the source.
For example, Steel and Iron are highly magnetic items so they send out a powerful field. This enables the user to determine whether to precede digging or not.
On the other hand gold is a lower magnetic pulse so would transmit a different frequency meaning those hours of digging are not endless.
With this type of metal detector you are almost bound to find those hidden treasures.
Pulse Indicator – This is one of the latest innovations of metal detecting. Usually used by door staff or guards to detect concealed weapons. It is not that efficient in detecting types of metals like the others. The portable device relies on echo or echolocation only. For example if you were to make a sound in an open room such as a hall the sound would repeat. If you were to do the same in a padded room the sound would be lost. Other detectors will use two coils, the portable detector only uses one or several that work as one.
An electric pulse is sent through the coils where it disappears and the magnetic field is reversed. This generates a short electric current and then disappears. It is the pulsing motion that detects any metal objects although it cannot determine what this maybe. For a portable detector it is one of the most expensive.
These are also called pinpoint detectors.
Industrial Metal Detectors – Better known as door-frame detectors. They are often found at airports or secure buildings such as a courthouse. It works solely on the principles of electromagnetism. Not one used for treasure hunting due to its size of course, but it is a great deterrent for criminals trying to enter the building with weapons.
The search coils are those based at the bottom of the detector. These come in many shapes and sizes. We are already aware one coil is used as a transmitter and the other is used as a receiver. It is also possible to have what is known as a “Monocoil”. This is where one coil will act as both the transmitter and receiver coil.
Not all, but some detectors will have the capability to have a “Tertiary coil”. This is a secondary coil which can assist with pinpointing the location of the artifact. Some detectors can even determine the depth of the item.
Metal Detector Coils
Size – Variation of coils can better the performance in different areas. Variations in sizes can mean not only more ground coverage but also deeper detection rates. It is (in general) known and accepted that the diameter of a coil is an approximation of its detection depth for a coin sized object although this is not always guaranteed.
Large coils – Expectedly search a larger scanning area, but this lead to it being less concentrated which can mean missing smaller items. It could also prove more difficult in larger areas where there are many metal items such as rubbish. With the many items inside it can be hard to pinpoint the treasure itself.
Small Coils – With smaller coils the magnetic field is more concentrated in smaller areas. Small coils work well in areas where there is a lot of metal debris. However due to the small diameter more scans may need to be done in the area. This is a positive though because it enables the user to possibly detect multiple targets.
Shapes – Coils come in different shapes and configurations that will offer different benefits based on the environment in which they are used. An elliptical coil has lot f flexibility than circular coils, and with its narrow width, it allows a much greater coverage of an area. A circular coil offers more sensitivity in soil which is none mineralized. It also has a great detection depth. This one is most commonly used shape on the market. Search coils can also come in a 2 box shape, this is best used for detecting deeply buried items, and this is due to the larger distant between transmitting and receiving.
What Are The 5 Different Types of Metal Detecting Search Coils?
Concentric Search Coils – The transmit coil is on the outside and the receiver coil is on the inside. These benefits the user as it gives the advantage of the coils being wound as large as possible in search diameter and depth detection.
They also provide the most symmetrical field; this makes it easier to pinpoint objects. It’s one of most common and versatile search coils because it gives consistency in target identification.
The design itself is not so flawless and is more susceptible to interference from ground minerals.
Imaging Coils – This is an advanced option of the concentric search coils. It is almost the same accept it has a second receive coil. This enables additional target information for true target sizing as well as target depths capabilities.
All the above allow the investor to differentiate trash from treasure of the same conductivity. This is only limited to one type of detector provide by one company.
Mono Coils – These coils are only available for Pule Induction detectors. The difference between this coil and the above coils is that the transmitter and receiver coils are located together. Or one coil which acts as both the transmitter and receive.
The mono coil provides maximum sensitivity but the performance in the mineralized ground.
Double D Coils – This design was built to reduce any ground interference, recovering performance which may be lost by the concentric coil in mineralized soil. The reason they are called Double D coils is because they are shaped like the letter D. With that set up the coils generate a cancelling effect of ground signals.
The positive detection field runs front to back beneath the centre of the design, the other part of the coil which that is negative, cancels the effect of the ground signals. This is what helps maintain the performance over the mineralized ground.
Due to the positive field being small the Double D design is less sensitive than the concentric design of the same size. However over mineralized ground the Double D is much better. This design would be used for such things as relic hunting.
2-Box Coils – In this design the receiver and transmit are separated by a quite a distance of feet. This search coil has a 3 to 4 foot diameter. The 2 box search coils are often used for deeply buried treasure such as relics. It is great for detecting larger objects but may find it difficult detecting smaller objects