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