How do metal detectors and other such devices work?

Thanks to The_Kenny for this subject. He wanted to know how a metal detector works, but I included some other devices.

My experience with the art of detecting stuff underground goes back many years. My father told me that you could use two bent coat hanger metal rods to detect water, pipes and other stuff underground. You simple cut the long straight section out of the metal hanger and then bend one side down. The long part is around 10 inches and the bent-at-ninety-degrees part is 4 to 5 inches. You hold the shorter parts in the cups of your hands so that they stick out horizontally about 5 inches apart. As you walk around, the two rods will slowly swing together when you pass over something underground. It works, but not everyone can make it work. This is a lot like the ancient art of dousing, which uses a so-called divining rod. There are scientific theories on how this works, but many scientists have debunked this idea.

A metal detector is, on the other hand, a proven concept, and there are many commercial devices out there. Airport security uses them as a means of detecting concealed weapons on passengers; archeologists use them to find ancient or old weapons and metal bullet fragments in battlefields; hobbyists use them to find old buried coins, especially gold and silver coins or jewelry on beaches and event sites; and fortune hunters also use them to detect iron-base meteorites, which are extremely valuable.

In order to understand how a metal detector works, one must realize that if an alternating electric current is passed through a wire coil, an alternating magnetic field is produced. The way to make an alternating electric current is to use an oscillator. An oscillator is the main electronic component in a radio transmitter. It produces a radio signal, or it can be used to create an alternating electrical signal. This information tells us how this idea was created. It was accidentally discovered by engineers trying to devise a way to find metal ore in rocks. Basically, a metal detector transmits a strong radio signal into a coil to produce a strong alternating magnetic signal, but that's on one half of the device. The other half involves detection.

When an alternating magnetic field is produced in a wire coil it induces what's called an eddy current in metal object that are close to the wire coil, or you could think of it as a transmitter. In other words the alternating magnetic signal induces a signal in the buried metal object, which produces its own alternating magnetic signal. This weaker induced magnetic signal can be detected by using a detector coil, or you could think of it as an antenna. The closer the metal object is, the stronger the induced eddy current and subsequent sympathetic magnetic field is. Most devices use the detector coil or antenna to produce an audio signal. The louder the audio signal, the closer the object is.

The polarity of the magnetic signal from the buried metal object is directly opposite to the transmitter's magnetic signal. In other words, the transmitted signal pushed down into the ground and pulses up and down. The induced signal from the metal object pulses up and down out of phase to the transmitted signal. That's what makes this idea possible because it allows the induced signal to be detected. Otherwise it would be swamped out by the stronger transmitted signal.

Most detectors simply put the smaller detector coil inside the larger transmitter coil. The two coils are adjusted so that they are in perfect electrical balance. If a metal object is near the detector, it disrupts the balance. Modern integrated circuits allow all of this to be refined and made much more sensitive while keeping the detector unit light and easy to use.
Use of these devices by hobbyists had created a problem. It seems that people are out there digging up archeological treasures, which is causing a conflict with professionals. It has resulted in the designation of many battlefield and historic sights as being off limits to hobby metal detector use.

Metal detectors are used to find antique bottles, toys, coins, jewelry, buried treasure, weapons of war, minerals that could contain diamonds and other precious stones, and geocaches. The last item is interesting because the term geocache literally means Earth hidden. Enthusiasts use GPS-enabled metal detectors to locate hidden treasures such as buttons, trinkets, toys or even books that have historical interest.

Metal detectors can also be used underwater as part of scuba diving expeditions. There are lots of treasures under the sea in the form of ancient treasure in sunken ships. This is a fascinating hobby for the adventure-seeking enthusiasts out there.

Thanks for reading.