metallic terminals were used without breaking the current, when carbon
was substituted the interval could be largely increased, and a light
of dazzling brilliancy appeared between the points.

This remarkable effect appears to be produced by the rarefaction of
the air, due to the great heat evolved by the combustion of the
carbon, and also to the passage of incandescent particles of carbon
from pole to pole, thus reducing the resistance, otherwise too great
for the current tension.

That was the beginning of electric lighting; and perhaps it will be
well to bridge the long and comparatively uninteresting interval which
elapsed between this discovery and the equally important one which
alone gave it commercial value--I refer to the production of suitable
currents by mechanical means. That is to say, the substitution of
energy obtained from coal in the form of steam power reduced the cost
to a fraction of what it necessarily was when the galvanic elements
were used. Here is the point; the cost of zinc today is something over
fifty times that of coal, while its energy as a vitalizing agent is
only about five times greater, leaving a very large margin in favor of
the "black diamonds." This is not the only advantage, for the
resulting impulse in the case of mechanical production is much more
uniform in action, and therefore better suited to the end in view,
while the amount of adjustment and attention required is beyond
comparison in favor of the latter means.

The machines adopted were of the magneto variety, and many ingenious
machines of this class were operated with more or less success, being,
however, quickly abandoned upon the introduction of the
dynamo-machine, which gave currents of much greater electromotive