simplest of expedients. When iron filings are sprinkled upon a card or
a sheet of glass below which a magnet is placed, the filings set
themselves--especially if aided by a gentle tap--along the lines of
force. Fig. 2 is a reproduction from nature of this very experiment, and
surpasses any attempt to draw the lines of force artificially. It
is impossible to magnetize a magnet without also in this fashion
magnetizing the space surrounding the magnet; and the space thus filled
with the lines of force possesses properties which ordinary unmagnetic
space does not possess. These lines give us definite information about
the magnetic condition of the space where they are. Their direction
shows us the direction of the magnetic forces, and their density shows
us the strength of the magnetic forces; for where the force is strongest
there we have the lines of force most numerous and most strongly
delineated in the scattered filings. To complete this first
consideration of the magnetic field surrounding a magnet, we will take a
look at Fig. 3, which reproduces the lines of filings as they settle in
the field of force opposite the end of a bar magnet. The repulsion of
the north pole of the magnet upon the north poles of other magnets would
be, of course, in lines diverging radially from the magnet pole.

[Illustration: Fig. 3]

We will next consider the space surrounding a wire through which a
current of electricity is flowing. This wire has magnetic properties so
long as the current continues, and will, like a magnet, act on a compass
needle. But the needle never tries to point toward the wire; its
tendency is always to set itself broadside to the current and at right
angles to it. The "field" of a current flowing up a straight wire is,
in fact, not unlike the sketch shown in Fig. 4, where instead of tufted
groups we have a sort of magnetic whirl to represent the lines of force.