production of dynamo-electric machines, but, in point of
fact, Faraday actually produced the first dynamo. A dynamo-electric
machine, as is well known, is a machine by means of which mechanical
energy is converted into electrical energy, by causing conductors to cut
through, or be cut through by, lines of magnetic force; or, briefly, it
is a machine by means of which electricity is readily obtained from
magnetism.

Faraday's invention of the first dynamo is interesting because at the
same time he made the invention he solved a problem which up to his time
had been the despair of the ablest physicists and mathematicians. This
was the phenomenon of Arago's rotating disc. It was briefly as follows:
If a copper disc be rotated above a magnet, the needle tends to follow
the plate in its rotation; or, if a copper plate be placed at rest above
or below an oscillating magnet, it tends to check its oscillations and
bring the needle quickly to rest. Faraday investigated these phenomena
and soon discovered that a copper disc rotated below two magnet poles
had electric currents generated in it, which flowed radially through the
disc between its circumference and centre. By placing one end of a
conducting circuit on the axis of the disc, and the other end on its
circumference, he succeeded in drawing off a continuous electric current
generated from magnetism, and thus produced the first dynamo. This was
in 1831. Faraday produced many other dynamos besides this simple
disc machine.

Although the disc dynamo in its original form was impracticable as a
commercial machine, yet it was not only the forerunner of the dynamo,
but was, in point of fact, the first machine ever produced that is
entitled to be called a dynamo. He generously left to those who might
come after him the opportunity to avail themselves of his wonderful
discovery. "I have rather, however," he says, "been desirous of
discovering new facts and new relations dependent on magneto-electric
induction than of exalting the force of those already obtained, being
assured that the latter would find their development hereafter." How
profoundly prophetic! Could the illustrious investigator see the
hundreds of thousands of dynamos that are to-day in all parts of the
world engaged in converting millions of horse-power of mechanical energy
into electric energy, he would appreciate how marvellously his
successors have "exalted the force" of some of the effects he had so
ably shown