f this, that is, the translation of the
energy of mechanical motion into that of an electrical current. In
addition to these primary functions which underlie the art of
telephony, the electromagnetic coil or helix serves a wide field of
usefulness in cases where no mechanical motion is involved. As
impedance coils, they serve to exert important influences on the flow
of currents in circuits, and as induction coils, they serve to
translate the energy of a current flowing in one circuit into the
energy of a current flowing in another circuit, the translation
usually, but not always, being accompanied by a change in voltage.

When a current flows through the convolutions of an ordinary helix,
the helix will exhibit the properties of a magnet even though the
substance forming the core of the helix is of non-magnetic material,
such as air, or wood, or brass. If, however, a mass of iron, such as a
rod or a bundle of soft iron wires, for instance, is substituted as a
core, the magnetic properties will be enormously increased. The reason
for this is, that a given magnetizing force will set up in iron a
vastly greater number of lines of magnetic force than in air or in any
other non-magnetic material.

Magnetizing Force. The magnetizing force of a given helix is that
force which tends to drive magnetic lines of force through the
magnetic circuit interlinked with the helix. It is called
_magnetomotive force_ and is analogous to electromotive force, that
is, the force which tends to drive an electric current through a
circuit.

The magnetizing force of a given helix depends on the product of the
current strength and the number of turns of wire in the helix. Thus,
when the current strength is measured in amperes, this magnetizing
force is expressed as ampere-turns, being the product of the number of
amperes flowing by the number of turns. The magnetizing force exerted
by a given current, therefore, is independent of anything except the
number of turns, and the material within the core or the shape of the
core has no effect upon it.

Magnetic Flux. The total magnetization resulting from a magnetizing
force is called the magnetic flux, and is analogous to current. The
intensity of a magnetic flux is expressed by the number of magnetic
lines of force in a square centimeter or square inch.

While the magnetomotive force or magnetizing force of a given helix is
independent of the material of the core, the flux which it sets up is