The Power of a Magnet is estimated by the weight its poles are able
to carry. Each pole singly is able to support a smaller weight than
when they both act together by means of a keeper, for which reason
horse-shoe magnets are superior to bar magnets of similar dimensions
and character. It has further been ascertained that small magnets have
a much greater relative force than large ones.
When magnetism is excited in a piece of steel in the ordinary mode, by
friction with a magnet, it would seem that its inductive power is able
to overcome the coercive power of the steel only to a certain depth
below the surface; hence we see why small pieces of steel, especially
if not very hard, are able to carry greater relative weights than large
magnets. Sir Isaac Newton wore in a ring a magnet weighing only 3
grains, which would lift 760 grains, _i. e._ 250 times its own weight.
Bar-magnets are seldom found capable of carrying more than their own
weight; but horse-shoe magnets of similar steel will bear considerably
more. Small ones of from half an ounce to 1 ounce in weight will carry
from 30 to 40 times their own weight; while such as weigh from 1 to 2
lbs. will rarely carry more than from 10 to 15 times their weight. The
writer found a 1 lb. horse-shoe magnet that he impregnated by means of
the feeder able to bear 26½ times its own weight; and Fischer, having
adopted the like mode of magnetising the steel, which he also carefully
heated, has made magnets of from 1 to 3 lbs. weight that would carry 30
times, and others of from 4 to 6 lbs. weight that would carry 20 times,
their own weight.--_Professor Peschel._