engine, or gas engine, or water-wheel. But of the thousands who have
heard that a steam engine can thus provide us with electric currents,
how many are there who comprehend the action of the generator or
dynamo-electric machine? How many, of engineers even, can explain where
the electricity comes from, or how the mechanical power is converted
into electrical energy, or what the magnetism of the iron magnets has
to do with it all? Take any one of the dynamo-electric machines of
the present date--the Siemens, the Gramme, the Brush, or the Edison
machine--of each of these there exist descriptions excellent in their
way, and sufficient for men already versed in the technicalities of
electric science. But to those who have not served an apprenticeship to
the technicalities--to all but professed electricians--the action
of these machines is almost an unknown mystery. As, however, an
understanding of the how and the why of the dynamo-electric machine or
generator is the very A B C of electrical engineering, an exposition
of the fundamental principles of the mechanical production of electric
currents demands an important place in the current science of the day.
It will be our endeavor to expound these principles in the plainest
terms, while at the same time sacrificing nothing in point of scientific
accuracy or of essential detail.

The modern dynamo-electric machine or generator may be regarded as
a combination of iron bars and copper wires, certain parts of the
machinery being fixed, while other parts are driven round by the
application of mechanical forces. How the movement of copper wires and
iron bars in this peculiar arrangement can generate electric currents is
the point which we are proposing to make clear. Friction has nothing
to do with the matter. The old-fashioned spark-producing "electrical
machine" of our youthful days, in which a glass cylinder or disk was
rotated by a handle while a rubber of silk pressed against it, has