ith
each other and with what is called a pole-changer. The whole of this
part is capable of revolving upon an axis P Y by a pulley at P. The
action is as follows: From their position, the soft-iron rods N' S'
must be magnets through the inductive action of the permanent magnet,
just as the nail was made a magnet in like position. So long as the
parts have the relative position shown in the figure, and there is no
motion, no electricity can be developed; but, if the axis P Y be turned,
S', which represents the polarity of the rod opposite N, will be losing
its induced magnetism; and, when half a revolution has been made, that
same pole will be where N' now is; but it will then have N' polarity
instead of S'; that is, it has been losing south polarity as it receded
from N, and gaining north polarity as it approached S: hence a current
of electricity has steadily been flowing through the coil in one
direction. At the same time, the other rod N' has passed through similar
phases; and its enveloping coil has had a current of electricity induced
in it in the same direction as in the first coil. This doubles the
intensity of the current; and the whole is conducted by the
connecting-wires where the current is wanted. Machines have been built
upon this plan, that contained fifty or sixty powerful compound
permanent magnets, and as many wire coils, needing a steam-engine of
eight or ten horse-power to run them.

A less cumbersome and much more efficient magneto-electric machine has
been made by changing the form of the soft iron armature to something
like a shuttle, and winding the wire inside of it. This is called the
"Siemen's Armature." The latest pattern of such machines is known as the
_Gramme_; and its peculiarity consists in the substitution of a broad
ring of soft iron for the armature. About this ring a good many coils,
of equal lengths, of insulated copper wire are wound in such a manner
that one-half of any turn in the wire goes through the inside of the
ring, making the coils longitudinal. The whole of the armature thus
prepared is fixed upon a shaft, so as to permit rotation, and fixed
between the poles of a powerful Jamin magnet. The ends of the coils are
connected with conductors upon the axis; and, when the armature thus
constructed is rotated, a very constant and powerful current of
electricity flows in a single direction, unlike the other forms. It is
stated, that, with one-horse power, a light can be obtained e