he
galvanometer wires was passed twice or thrice loosely round the brass
axis of the plate, and the other attached to a conductor, which itself
was retained by the hand in contact with the amalgamated edge of the
disk at the part immediately between the magnetic poles. Under these
circumstances all was quiescent, and the galvanometer exhibited no
effect. But the instant the plate moved the galvanometer was influenced,
and by revolving the plate quickly the needle could be deflected ninety
degrees or more."(4)

This rotating disk was really a dynamo electric machine in miniature,
the first ever constructed, but whose direct descendants are the
ordinary dynamos. Modern dynamos range in power from little machines
operating machinery requiring only fractions of a horsepower to great
dynamos operating street-car lines and lighting cities; but all
are built on the same principle as Faraday's rotating disk. By this
discovery the use of electricity as a practical and economical motive
power became possible.

STORAGE BATTERIES

When the discoveries of Faraday of electro-magnetic induction had made
possible the means of easily generating electricity, the next natural
step was to find a means of storing it or accumulating it. This,
however, proved no easy matter, and as yet a practical storage or
secondary battery that is neither too cumbersome, too fragile, nor too
weak in its action has not been invented. If a satisfactory storage
battery could be made, it is obvious that its revolutionary effects
could scarcely be overestimated. In the single field of aeronautics, it
would probably solve the question of aerial navigation. Little wonder,
then, that inventors have sought so eagerly for the invention of
satisfactory storage batteries. As early as 1803 Ritter had attempted to
make such a secondary battery. In 1843 Grove also attempted it. But it
was not until 1859, when Gaston Planche produced his invention, that
anything like a reasonably satisfactory storage battery was made.
Planche discovered that sheets of lead immersed in dilute sulphuric acid
were very satisfactory for the production of polarization effects. He
constructed a battery of sheets of lead immersed in sulphuric acid, and,
after charging these for several hours from the cells of an ordinary
Bunsen battery, was able to get currents of great strength and
considerable duration. This battery, however, from its construction of
lead, was necessarily heavy and cum