A metal detector coil is just that, an electronic coil. It is the round flat object at the end of the detectors rod or wand. Many of these are actually coils of wire, although the newer coils are flat printed-circuit boards with the electronic pathways in circular form, a new and more powerfully efficient type of coil. When electricity is passed through the metal detectors coil it creates a toroid aura which surrounds said coil. Toroid means donut shaped. Technically the emanations of this electronic field in the shape of a donut are said to be infinite, but the coils measurable power is only a matter of inches, or a few feet at most, from the coil itself. And this electronic aura, like other electronic fields, is capable of induction, that is, of inducing a voltage, or more correctly, a current, in nearby metallic objects. These induced currents take on the same circular form as the coil, and on the surface of the metallic object they form what are called eddie currents. The eddie currents react with the aura/field of the metal detectors coil in measurable ways. This reaction of the eddie currents with the coils electronic aura/field creates a feedback of sorts, which the electronics in the metal detectors box can then interpret to create audio signals recognizable to the human ear.
The ability of a metal detector to discern between ferrous and non-ferrous metals is known as Discrimination. Whites Electronics of Sweet Home Oregon patented some of the first discrimination circuits ever known, which are actually electronic recognition circuits. These specialised circuits use a phase analyzing attribute to produce the ability of Discrimination in their metal detectors. This basically means the electronics in the metal detectors control box are capable of interpreting the basic feedback signal from the coil in at least two different ways; the differences in the signals indicates whether a material is ferrous (iron) or not, and the subsequent audio signal reflects this difference so that a user may better judge what the aura of his coil is reacting with.