The ends of the coil were next connected to the external circuit through a “commutator.”  This is a device which is arranged to convert the alternating current of the coils into a current of one direction in the external circuit, and which in some form is found on all direct-current dynamos.  Joseph Saxton, an American, improved upon Pixii’s machine by rotating 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 will come no one will question; but where is the daring prophet who will hazard a guess as to what they will be?