How Metal Detectors Work
The basic operation of any metal detector is to transmit, receive, and discriminate. But with new technology you have underwater metal detectors, gold prospecting metal detectors, metal detectors that can discriminate between gold and silver coins and jewelry.
Inside the search head of a metal detector is a wire coil called a transmit coil. Electricity is sent through the coil to create an electromagnetic field.
The direction of the electricity is reversed thousands of times per second. When the electricity moves in one direction, a magnetic field is created whose polarity is pointed outward, or in this case to the ground. When the polarity is reversed, it sends a signal back to the machine. If there is metal in the ground, the electrical current will pass through the metal, allowing it to create its own magnetic field, which will work opposite of the transmit signal, forcing itself back to the detector.
A second coil inside the search head is arranged so that all the currents sent back by the transmit coil will be wiped out. This way, it can pick up only the current sent by the magnetic field of the metal object and receive only that object.
Once the electrical current has been transmitted and received, the simplest form of discrimination allows the metal detector to respond with an audio output when it passes over a metal object. Each different type of metal will have a signal unique to its own properties.
As a result, the machine will then be able to distinguish between, for example, a quarter made of silver and copper and an aluminum can tab. The metal detector can then be adjusted in sensitivity so that it responds only to heavier metals.