By arranging the coil so as to form an electro-magnet, as before
described, and holding the magnet under a plate of glass sprinkled
with fine iron filings, as shown in Fig. 15, and then sending a
current through the magnet, at the same time jarring the glass by
striking it with a lead pencil, a magnetic figure will be formed which
is sometimes called the magnetic spectrum. By connecting the terminals
of the coils diagonally with each other, and connecting the remaining
terminals with the battery, two like poles will be formed, and the
magnetic figures will have an entirely different appearance, owing to
the repulsion between the two like polarities. Different figures may
be produced by using the solenoids without the iron cores.