The technology behind all
metal detectors is the same. All metal detectors are variations
on the same theme, which is electromagnetic . When you put electricity
into a coil of wire, a magnetic field is created, and any piece
of metal within the near vicinity of that magnetic field also starts
to radiate its own magnetic field. That magnetic field will hang
around for a microsecond or two, even when the electricity to the
coil is switched off. A second coil can be tuned to detect that
tiny magnetic field. And that's how the metal detector works. The
electronics in a metal detector converts battery power into a signal
of a certain frequency, or several frequencies in the case of a
multi frequency machine. These signals are radiated from the transmit
coil in the search-head and metal objects within range of the signal
transmission will be magnetised in the way described above. A second
receive coil fitted in the search-head next to the transmit coil
is 'listening' for the residual signals coming from metal objects
in the ground.
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