f the motion which acts upon it. When the wheel of a common
electrical machine is turned, the product is electricity,--a force very
different from that which originates it. Ordinary mechanical motion
_goes in_; electricity _comes out_, the latter being a modified motion
due to the physical structure of the machine. In like manner, a magnet
may be considered as a machine by means of which mechanical motion may
be converted into some other form of motion. It is evident that
molecular structure is chiefly concerned in this. If a bar of iron that
exhibits no evidence of magnetism whatever be subjected to torsion, it
will immediately become a magnet with poles dependent upon the direction
of the twist. This developed magnetism will re-act upon a coil of wire,
and so move a galvanometer needle. If the bar be permitted to recover
its original condition, it will lose its magnetism, which will at once
re-appear upon twisting the rod again. Now, when the rod is twisted, it
is evident that there is a molecular strain in certain directions
throughout the mass. The converse experiment illustrates the same thing.
It has been found, that when a rod of iron is made magnetic by the
action of a current of electricity circulating about it, and at the same
time passing longitudinally through it, the rod is slightly lengthened
and twisted in a direction that depends upon the direction of the
current. Moreover, if a permanent magnet be heated to a red heat, its
magnetism is destroyed; for such a heat allows the molecules to freely
arrange themselves without any external constraint. Also, if a permanent
magnet be suspended so as to give out a musical sound when it is struck,
the magnetism will be much weakened by making it thus to vibrate. In
this case, as in the other, the vibrations affect every molecule, and so
enable them to re-adjust themselves to the positions they held before
being magnetized. The same thing happens when a bar of iron is made
magnetic through the inductive action of the earth. When this bar is
held in the direction of the magnetic dip, it becomes but very slightly
magnetized; but, if it be so held that when it is struck with a hammer
it will ring, that is, give out a musical sound, it will at once become
decidedly magnetic. Evidently the earth's action tends to set the
molecules of the mass in a new position, but cohesion prevents them from
assuming it. When the molecules are made to vibrate, they can assume
such new pos