If you've flown on an airplane in the last 30 years, you're familiar with metal detectors. You probably also recognize them on the beach - some intrepid individual diligently combing the beach for lost items. Metal detecting is a pretty popular hobby, and security metal detectors are being installed in more and more locations. Metal detectors are also used for prospecting metals like gold, silver, and iron. All of these applications of metal detectors rely on essentially the exact same machinery.
How a metal detector works First, some theory: Metal detectors work on the principle of electromagnetic or just "magnetic" induction. When a conductor interacts with a magnetic field, it starts to give off some electricity. This causes the conductor within the field to begin generating its own magnetic field. Somewhere within each metal detector, whether for hobby or security, lies a large coil of copper wire. The metal detector generates a magnetic field when it sends electricity from its power source through the copper coil. Any conductor, such as an old coin, a gold nugget, or a knife, passing through the metal detector’s magnetic field would be affected, beginning to produce electricity and generate a new magnetic field. The metal's magnetic field, in turn, produces an effect on the detector's coil, and the resulting opposite flow of current through the device is recognized the operator of the device is then notified that metal has, indeed, been detected. History of the metal detector The early days The metal detector is a pretty old idea - it was pursued eagerly since the early days of electric theory in the late 1800s. The appeal was clear - a portable device that could tell miners exactly where they would find precious metals buried in the earth? Soon, other uses were thought of - a Parisian named Gustave Trove designed a handheld metal detector for pinpointing the locations of bullets inside of soldiers for extraction. Alexander Graham Bell, most famous for inventing the telephone, produced a working version of this to detect the bullet that was used to assassinate President James A. Garfield. Development A much more modern version of the device was used in the 192os to recover lost treasures from the Emperor Caligula’s ships, and in the 193os to recover the supplies of past Antarctic explorers. During World War ll, the Polish army developed the same device into a mine detector; this was the first practical metal detector designed for individual use. The growing apparent practicality of metal detecting attracted many manufacturers to create their own versions, refining the product into what we know today. Metal detectors today Security Today, the uses of metal detectors are many and varied. They're ubiquitous at airports, and have been since a string of airplane hijackings in the early 1970's prompted their installation in US airports. A Finnish metal company saw the opportunity to adapt metal detectors for mining into large, walk through detectors for security. Eventually, companies arose designing metal detectors specifically for airports and other secure locations. Innovations have led to improved detection capabilities, including the ability to determine approximately how far away from the ground the detected object is - aiding security personnel. Hobby As individual metal detectors were made more affordable, more and more consumers purchased them for personal use. Beach combing is a popular hobby usage of metal detectors, as is the practice of looking for old (or even current) coins a practice known as "coin shooting”. Hobbyists are also known to set out prospecting for metals in the earth to make a bit of profit. A more controversial use of metal detectors involves the disturbance of materials of archaeological significance. Archaeology Metal detectors can be employed by archaeologists to locate past cultural sites rich with artifacts – but individuals can use them in the same way. People will often be found searching for physical remains of past battles at known battle sites. In many places, the laws on metal detecting are fuzzy some countries have special laws governing the use of metal detectors and the ownership of metals found. A recent stunning archaeological discovery was made in Stafford shire, England by a man with a metal detector. He was combing an area of fresh farmland for whatever he might find, and stumbled upon the largest Anglo-Saxon hoard of gold and silver ever found - now known as the Stafford-shire Hoard. A metal detector-centric law specified that the landowner is paid the full valuation of the hoard by all museums wishing to display it. Prospecting Human artifacts aren't the only valuable things metal detector enthusiasts hope to find. Some set out looking for natural metal deposits in the earth - typically valuable ones like gold. The largest gold nugget ever found in the United States was discovered in California by a man with a metal detector. Kinds of metal detector Beat Frequency Oscillation (BFO) This is the most commonly used type of metal detector for personal use among beginners, owing to its simplicity. In this form, a metal detector consists of two metal coils, connected to oscillators running at different frequencies. Speakers, typically connected to an earpiece, pick up the sound of the interacting oscillators, which is typically a steady hum. However, when the field is disturbed by the presence of a metal, the interference caused by a new electromagnet produces an audible effect. Very Low Frequency (VLF) This kind, like the BFO metal detector, uses two separate, concentric metal coils. The outside coil sends out a very low-frequency field, and the inside coil amplifies the noise resulting from metal objects. VLF metal detectors also include a small on board computer, known as a tuner. It's programmed to accept signals only past a certain threshold, or within a certain range as different metals give off different signals, this allows operators to choose which metals they're looking for, with the detector passing right over undesirable metals. Pulse Induction (Pl) These are the most recently invented variety metal detectors. These can detect metals deep beneath the earth or water. Large versions of these are the current standard in security metal detectors. Pulse induction detectors, unlike BFO and VLF detectors, consist of only one coil. This coil acts as both the transmitter and the receiver. Instead of oscillation, short bursts of energy (that is, pulses), typically about 100 per second, but sometimes as high as 1000. When a metal object is within the pulse field, each pulse takes a little longer to disappear, due to the reflected magnetic field generated by the object adding to the original pulse field. The length of each pulse is measured by what's known as a sampling circuit. The sampling circuit is connected to an integrator hooked up to an audio circuit - when the integrator is informed of a Ionger than normal pulse, it generates DC current through the audio circuit, producing an audible tone. These are less popular for use among prospectors, due to the inability to easily discriminate between metals.
1 Comment
8/11/2023 04:32:00 am
The sampling circuit is connected to an integrator hooked up to an audio circuit when the integrator is informed of a Ionger than normal pulse. Thank you for sharing your great post!
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