n electric stream. A third and remarkable vista opens
to experiment when it deals with substances which, in their normal
state, are non-conductive, but which, agitated by an electric wave,
instantly become conductive in a high degree. As long ago as 1866 Mr. S.
A. Varley noticed that black lead, reduced to a loose dust, effectually
intercepted a current from fifty Daniell cells, although the battery
poles were very near each other. When he increased the electric tension
four- to six-fold, the black-lead particles at once compacted themselves
so as to form a bridge of excellent conductivity. On this principle he
invented a lightning-protector for electrical instruments, the incoming
flash causing a tiny heap of carbon dust to provide it with a path
through which it could safely pass to the earth. Professor Temistocle
Calzecchi Onesti of Fermo, in 1885, in an independent series of
researches, discovered that a mass of powdered copper is a non-conductor
until an electric wave beats upon it; then, in an instant, the mass
resolves itself into a conductor almost as efficient as if it were a
stout, unbroken wire. Professor Edouard Branly of Paris, in 1891, on
this principle devised a coherer, which passed from resistance to
invitation when subjected to an electric impulse from afar. He enhanced
the value of his device by the vital discovery that the conductivity
bestowed upon filings by electric discharges could be destroyed by
simply shaking or tapping them apart.

In a homely way the principle of the coherer is often illustrated in
ordinary telegraphic practice. An operator notices that his instrument
is not working well, and he suspects that at some point in his circuit
there is a defective contact. A little dirt, or oxide, or dampness, has
come in between two metallic surfaces; to be sure, they still touch each
other, but not in the firm and perfect way demanded for his work.
Accordingly he sends a powerful current abruptly into the line, which
clears its path thoroughly, brushes aside dirt, oxide, or moisture, and
the circuit once more is as it should be. In all likelihood, the coherer
is acted upon in the same way. Among the physicists who studied it in
its original form was Dr. Oliver J. Lodge. He improved it so much that,
in 1894, at the Royal Institution in London, he was able to show it as
an electric eye that registered the impact of invisible rays at a
distance of more than forty yards. He made bold to say that this