the coils, or armature as it is called,
and making the heavier magnet stationary. The essential points of
construction being worked out, improvements followed rapidly. Dr.
Werner Siemans, of Berlin, introduced an important modification by
making the revolving armature of a cylinder of soft iron, having a
groove cut throughout its length on opposite sides. In these grooves a
wire was wound and the armature was rotated on its axis between the
poles of several magnets.

In all the earlier machines permanent magnets of steel were used. The
next important step was to use electro-magnets of soft iron, excited
by a current flowing through many turns of wire wound around the legs
of the magnet. These could be made much more strongly magnetic than
the permanent magnets. The exciting current was at first obtained from
a small permanent magneto machine; but it was afterward found that the
machine could be made self-exciting. Soft-iron electro-magnets, after
being once magnetized, remain slightly magnetic. This will produce a
weak current in the revolving armature which is turned into the magnet
coils. The magnets are thus further magnetized, and again react upon
the armature with greater intensity. In this way a _strong_ current is
rapidly built up, and after wholly or in part passing around the
magnet coils to sustain its magnetism, can be carried out into the
circuit to serve the great variety of purposes to which it is now put.

The essential points in the evolution of the dynamo can here be
sketched only in broadest outline. Even to catalogue in detail, the
improvements of Edison and Brush, Gramme and Wheatstone, and a host of
others who have contributed to the work, would require a volume. One
fact, however, should ever be kept in mind: Whatever may be the extent
of the superstructure of electrical science, it is all built upon the
foundation of electro-magnetic induction laid by Michael Faraday. The
little "magnetic spark" he first produced, and the trembling of his
galvanometer-needle, were but signals of the birth of the giant of the
century.

These are the days of electricity and steel, and a fitting part of the
intense age in which they exist. That we have as yet seen but a
partial development of the possibilities of the electrical discovery,
no one can doubt. The rush of the trolley car, and the blinding flash
of the electric light, are but challenges thrown out to the future for
even greater achievements. That they