Magnetizing Force
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
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
wholly of iron or steel, which are highly magnetic substances.
Permeability. The quality of material, which permits of a given
magnetizing force setting up a greater or less number of lines of
force within it, is called its permeability. More accurately, the
permeability is the ratio existing between the amount of magnetization
and the magnetizing force which produces such magnetization.
The permeability of a substance is usually represented by the Greek
letter µ (pronounced _mu_). The intensity of the magnetizing force
is commonly symbolized by H, and since the permeability of air is
always taken as unity, we may express the intensity of magnetizing
force by the number of lines of force per square centimeter which it
sets up in air.
Now, if the space on which the given magnetizing force H were acting
were filled with iron instead of air, then, owing to the greater
permeability of iron, there would be set up a very much greater number
of lines of force per square centimeter, and this number of lines of
force per square centimeter in the iron is the measure of the
magnetization produced and is commonly expressed by the letter =B=.
From this we have
µ = B/H
Thus, when we say that the permeability of a given specimen of wrought
iron under given conditions is 2,000, we mean that 2,000 times as many
lines of force would be induced in a unit cross-section of this sample
as would be induced by the same magnetizing force in a corresponding
unit cross-section of air. Evidently for air B = H, hence µ becomes
unity.
The permeability of air is always a constant. This means that whether
the magnetic density of the lines of force through the air be great or
small the number of lines will always be proportional to the
magnetizing force. Unfortunately for easy calculations in
electromagnetic work, however, this is not true of the permeability of
iron. For small magnetic densities the permeability is very great, but
for large densities, that is, under conditions where the number of
lines of force existing in the iron is great, the permeability becomes
smaller, and an increase in the magnetizing force does not produce a
corresponding increase in the total flux through the iron.
Magnetization Curves. This quality of iron is best shown by the curves
of Fig. 89, which illustrate the degree of magnetization set up in
various kinds of iron by different magnetizing forces. In these curves
the ordinates represent the total magnetization =B=, while the abscissas
represent the magnetizing force =H=. It is seen from an inspection of
these curves that as the magnetizing force =H= increases, the intensity
of flux also increases, but at a gradually lessening rate, indicating a
reduction in permeability at the higher densities. These curves are also
instructive as showing the great differences that exist between the
permeability of the different kinds of iron; and also as showing how,
when the magnetizing force becomes very great, the iron approaches what
is called _saturation_, that is, a point at which the further increase
in magnetizing force will result in no further magnetization of the
core.