146 PHYSICS. 
developed in them. For this reason tempered steel is generally employed, 
although furnishing weaker magnets. 
The magnetic force is completely destroyed by great heat, and cannot 
again be restored in natural magnets. In ‘artificial magnets this may be 
adhe by again hardening the Sect The limit of temperature beyond which 
bodies are unsusceptible of the influence of the magnet, varies with the 
material. This, for manganese, lies between 65° and 70° F-., for nickel at 
about 662° F., and for cobalt far above a white heat. 
Light possesses the power of magnetizing a steel needle. This property 
is nearly confined to the violet ray, being slightly shared, however, by the 
blue and green. Only that half of the needle which is to become the north 
pole must be exposed to the influence of the ray, it being necessary carefully 
to cover the other. A needle will even be magnetized by laying a plate of 
glass, colored blue or green with cobalt, over the north pole. and exposing 
the whole to the solar rays. ‘The same end will also be accomplished by 
wrapping the north pole with blue or green bands, and placing the whole 
for some days in the sun. 
An armature is necessary to retain the magnetism of an artificial magnet 
for any length of time. This is a piece or plate of soft iron, so constructed 
as to connect the poles of the magnet, thus becoming not only magnetic 
itself, but reciprocally causing the development of fresh magnetism in the 
magnet. In fig. 19, pp' represents the armature of a horse-shoe magnet. 
The ring nn’ serves to suspend the magnet. The armature of natural 
magnets is exhibited in figs. 20,21. Here //’ are the wings, pp’ the feet of 
the armature ; the former are nearly as broad as the magnet, and about one 
line thick. 
A magnetic battery, required whenever a great degree of magnetism is 
wanted, is formed by the combination of a number of single magnets with 
their like poles placed together. Fig. 19 is a horse-shoe battery; the one 
represented in fig. 16 is the form recommended by Coulomb. This consists 
of twelve bars, disposed in three layers of four bars each. The bars of the 
central layer are from two and a half to three inches longer than the others, 
which are of equal length. The bars are al] fastened in pieces of iron, f, 
which serve both for armatures and feet to the compound magnet. The 
whole apparatus is held compactly together by the brass bands cc’. 
B. Execrricrry. 
1. Of Electrical Actions. 
Many bodies, as glass, resin, sulphur, amber, &c., exhibit the property of 
attracting light objects on being rubbed. If the friction be sufficiently 
violent, and the proper rubber be used, a spark will pass from one of these 
substances to the knuckle. The cause of these curious phenomena is 
assumed to lie in the existence of a peculiar fluid or fluids called electricity, 
pervading all bodies. 
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