will answer as well as the magnetic needle; for the needle of iron is
magnetized by the magnet, and acts exactly like a steel needle
independently magnetized.

If we place two or more needles of iron near the magnet, the action
becomes more complex, for then the needles are not only acted on by
the magnet, but they act upon each other. And if we pass to smaller
masses of iron--to iron filings, for example--we find that they act
substantially as the needles, arranging themselves in definite forms,
in obedience to the magnetic action.

Placing a sheet of paper or glass over a bar magnet and showering iron
filings upon the paper, I notice a tendency of the filings to arrange
themselves in determinate lines. They cannot freely follow this
tendency, for they are hampered by the friction against the paper.
They are helped by tapping the paper; each tap releasing them for a
moment, and enabling them to follow their tendencies. But this is an
experiment which can only be seen by myself. To enable you all to see
it, I take a pair of small magnets and by a simple optical arrangement
throw the magnified images of the magnets upon the screen. Scattering
iron filings over the glass plate to which the small magnets are
attached, and tapping the plate, you see the arrangement of the iron
filings in those magnetic curves which have been so long familiar to
scientific men (fig. 23).

[Illustration: Fig. 23.

N is the nozzle of the lamp; M a plane mirror, reflecting the beam
upwards. At P the magnets and iron filings are placed; L is a lens
which forms an image of the magnets and filings; and R is a totally