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.

FIG. 15. - MAGNETIC FIGURES.