Page:Popular Science Monthly Volume 59.djvu/355

Rh strokes it with a natural magnet and feels certain that he has magnetized it; and he assures us that 'one of the poles will be at the point rubbed and the other will be at the opposite side;' and how does he convince himself that the ring is really magnetized? He cuts it across at the point opposite the one rubbed, opens it out, and finds that the ends exhibit polar properties.

A favorite piece of apparatus with Gilbert was a lodestone ground down into globular form. He called it a terrella, a miniature earth. He used it extensively for reproducing the phenomena described by magnetizers, travelers and navigators as observed in their compass needles. He breaks up terrellas, in order to examine the magnetic condition of their inner parts. There is not a doubtful utterance in his description of what he finds; he speaks clearly and emphatically. "If magnetic bodies be divided, or in any way broken up, each several part hath a north and a south end;" i. e., each part will be a complete magnet.

We find him also comparing magnets by what is known to us as the 'magnetometer method.' He brings the magnetized bars in turn near a compass needle and concludes that the magnet or the lodestone which is able to make the needle go round is the best and strongest. He also seeks to compare magnets by a process of weighing, similar to what is called, in laboratory parlance, the 'test-nail' method. He also inquires into the effect of heat upon his magnets, and finds that 'a lodestone subjected to any great heat loses some of its energy.' He applies a red-hot iron to a compass needle and notices that it 'stands still, not turning to the iron.' He thrusts a magnetized bar into the fire until it is red-hot and shows that it has lost all magnetic power. He does not stop at this remarkable discovery, for he proceeds to let his red-hot bars cool while lying in various positions, and he finds: (1) that the bar will acquire magnetic properties if it lie in the magnetic meridian; and (2) that it will acquire none if it lie east and west. These effects he rightly attributes to the inductive action of the earth.

Gilbert marks these and other experiments with marginal asterisks; small stars denoting minor and large ones important discoveries of his. There are in all 21 large and ITS small asterisks, as well as 84 illustrations in De Magnete. This implies a vast amount of original work, and forms no small contribution to the foundations of electric and magnetic science.

Gilbert clearly realized the phenomena and laws of magnetic induction. He tells us that "as soon as a bar of iron comes within the lodestone's sphere of influence, though it be at some distance from the lodestone itself, the iron changes instantly and has its form renewed; it was before dormant and inert; but now is quick and active." He hangs a nail from a lodestone; a second from the first, a third from the