478 MAGNETISM AND ELECTRICITY iron alloy containing aluminum, nickel, and cobalt, hence called "alnico," is so powerfully magnetic when magnetized that it lifts 500 times its own weight. An alnico magnet constructed with many air gaps has supported 4450 times its own weight. Another alloy composed of vanadium, iron, and cobalt, developed by the Bell Telephone Laboratories, will hold more permanent magnetism than any other known material. A whole range of alloys of iron, nickel, chromium, and silicon can be prepared in such proportions that they will lose or regain their magnetism at certain definite temperatures from — 150° C. to 1100° C, thus providing a new method of producing automatic temperature controls. The first serious study of magnetism seems to have been made by the physician, Sir William Gilbert (1540-1603), at the end of the sixteenth century. A unit of magnetism, the "gilbert," has been named after him. He noted that in England the needle of the mariner's compass dipped with its north pole downwards through an angle depending on the latitude, and he inferred from these experiments that the earth itself acts as a huge magnet, with its poles considerably distant from the geographical poles. Gilbert observed that the attraction of a magnet appears to be concentrated at tw^o points which are called poles. In the case of bar magnets the poles are generally near the ends, as can be proved by dipping them in iron filings. William Gilbert is considered to have been the founder of the sciences of magnetism and electricity. Francis Bacon repeatedly referred to him as one of the first men to practice the experimental method. Like Poles Repel and Unlike Poles Attract Each Other. Gilbert made a small globe out of lodestone and found that it behaved much like the earth toward compasses. He found that what we call the north magnetic pole of the earth corresponds to the south pole of a magnet; north-seeking poles of magnets repel each other, while there is attraction between Fig. 223. Like poles are repelled unlike poles. This is a very impor- ^"^^^ ^'^ attracted by ^^^^ j^^ ^^ magnetism, namely, e'a^ch'othei-''^ attract each other. that like poles repel and unlike poles The amount of their attraction or repulsion varies with the strength of the poles and inversely with the square of the distance between them — another example of the inverse square law. The north pole of a magnet is the end of the magnet that points toward the north pole of the earth.
MAGNETISM 479 Magnets Are Surrounded by Magnetic Fields. The fact that it is not necessary for magnets to touch each other in order for their attractive or repulsive force to act is accounted for by the hypothesis that the space around a magnet must be in a state of strain. Fig. 224. A floating magnet. One magnet is held in position above the other magnet, because of the repulsion of like poles. The region subject to this strain is called the magnet's field, or simply the magnetic field. A very good map of such a field can be obtained by placing a piece of paper over a magnet and then sprinkling iron filings over the paper; see Fig. 225. It will be noted that the complete lines of force (used to represent a magnetic field) extend from the north pole to the south pole or vice versa, and this would be found true of every line if one could follow it far enough, as indicated in Fig. 226. Michael Faraday (1791-1867), the Fig. 225. Lines of force of a rod experimenter who did more than any magnet as shown by iron filings other man to place the science of P'^^^^ o" ^ ^^^^^ of P^P^'' ^^^^^ magnetism and electricity upon a firm ^ ^ basis, found that these lines of force behave like stretched rubber bands, trying to shorten in the direction of their length and to widen in the direction of their width. The Earth Is a Giant Magnet. The poles of an ordinary magnet derive their names from their behavior toward the earth. Those that point north are called north poles. As we have already pointed out, the magnetic poles and geographic poles do not coincide.