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
largely dependent on the material and shape of the core--not only upon
this but on the material that lies in the return path for the flux
outside of the core. We may say, therefore, that the amount of flux
set up by a given current in a given coil or helix is dependent on the
material in the magnetic path or magnetic circuit, and on the shape
and length of that circuit. If the magnetic circuit be of air or brass
or wood or any other non-magnetic material, the amount of flux set up
by a given magnetizing force will be relatively small, while it will
be very much greater if the magnetic circuit be composed in part or