Such transient currents are practically useless, and several devices
have been invented to make the flow continuous. The common form of
machine for doing this may be understood by reference to the diagram.
[Illustration: FIG. 4.]
N S, Fig. 4, is the permanent magnet, which is bent into a U form in
order to utilize both poles. N' and S' are short rods of soft iron
fastened into a yoke-piece Y, also of soft iron. Coils of wire surround
each of the rods as represented, the ends of the wires connecting with
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 equal to
that from a battery of fifty Grove cells.