Such
lines of force run through the magnetic "field" from one pole of the
magnet to the other in curves. If we define a line of force as being the
line along which a free north-seeking magnetic pole would be urged, then
these lines will run from the north pole of the magnet round to the
south pole, and pass through the substance of the magnet itself. In Fig.
1 a rough sketch is given of the lines of magnetic force as they emerge
from the poles of a bar magnet in tufts. The arrow heads show the
direction in which a free north pole would move. These lines of forces
are no fiction of the imagination, like the lines of latitude and
longitude on the globe; they exist and can be rendered visible by the
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.
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