Man's physical universe

xanabras

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.

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