Transmitter
Inside the metal detector's loop (sometimes
called a search head, coil, antenna, etc.) is a coil of wire
called the transmit coil. Electronic current is driven through
the coil to create an electromagnetic field. The direction of
the current flow is reversed several thousand times every
second; the transmit frequency "operating frequency" refers to
the number of times per second that the current flow goes from
clockwise to counterclockwise and back to clockwise again.
When the current flows in a given direction,
a magnetic field is produced whose polarity (like the north
and south poles of a magnet) points into the ground; when the
current flow is reversed, the field's polarity points out of
the ground. Any metallic (or other electrically conductive)
object which happens to be nearby will have a flow of current
induced inside of it by the influence of the changing magnetic
field, in much the same way that an electric generator
produces electricity by moving a coil of wire inside a fixed
magnetic field. This current flow inside a metal object in
turn produces its own magnetic field, with a polarity that
tends to be pointed opposite to the transmit field.
Receiver
A second coil of wire inside the loop, the
receive coil, is arranged (by a variety of methods) so that
nearly all of the current that would ordinarily flow in it due
to the influence of the transmitted field is cancelled out.
Therefore, the field produced by the currents flowing in the
nearby metal object will cause currents to flow in the receive
coil which may be amplified and processed by the metal
detector's electronics without being swamped by currents
resulting from the much stronger transmitted field.
The resulting received signal will usually
appear delayed when compared to the transmitted signal. This
delay is due to the tendency of conductors to impede the flow
of current (resistance) and to impede changes in the flow of
current (inductance). We call this apparent delay "phase
shift". The largest phase shift will occur for metal objects
which are primarily inductive; large, thick objects made from
excellent conductors like gold, silver, and copper. Smaller
phase shifts are typical for objects which are primarily
resistive; smaller, thinner objects, or those composed of less
conductive materials.
Some materials which conduct poorly or not
at all can also cause a strong signal to be picked up by the
receiver. We call these materials "ferromagnetic".
Ferromagnetic substances tend to become magnetized when placed
in a field like a paper clip which becomes temporarily
magnetized when picked up with a bar magnet. The received
signal shows little if any phase shift. Most soils and sands
contain small grains of iron-bearing minerals which causes
them to appear largely ferromagnetic to the metal detector.
Cast iron (square nails) and steel objects (bottle caps)
exhibit both electrical and ferromagnetic properties.
It should be pointed out that this
discussion describes an "Induction Balance" metal detector,
sometimes referred to as "VLF" Very Low Frequency (below
30kHz). This is the most popular technology at the present
time, and includes the "LF" Low Frequency (30 to 300kHz)
instruments made for prospecting.
