city is accomplished in several ways. In general,
however, the principle by which the result is accomplished is one, and
depends upon the resistance which the electrical current meets in its
transmission through various substances. There are no perfect
conductors of electricity. In proportion as the non-conductive quality
is prevalent in a substance, especially in a metal, the resistance to
the passage of electricity is pronounced, and the consequent
disturbance among the molecular particles of the substance is great.
Whenever such resistance is encounted in a circuit, the electricity is
converted into heat, and when the resistance is great, the heat is, in
turn, converted into light, or rather the heat becomes phenomenal in
light; that is, the substance which offers the resistance glows with
the transformed energy of the impeded current. Upon this simple
principle all the apparatus for the production of electric light is
produced.

Among the metallic substances, the one best adapted by its low
conductivity to such resistance and transformation of force, is
platinum. The high degree of heat necessary to fuse this metal adds to
its usefulness and availability for the purpose indicated. When an
electrical current is forced along a platinum wire too small to
transmit the entire volume, it becomes at once heated--first to a red,
and then to a white glow--and is thus made to send forth a radiance
like that of the sun. Of the non-metallic elements which offer similar
resistance, the best is carbon. The infusibility of this substance
renders it greatly superior to platinum for purposes of the electric
light.

Near the beginning of the present century it was discovered by Sir
Humphry Davy that carbon points may be rendered incandescent by means
of a powerful electrical current. The discovery was fully developed in
the year 1809, while the philosopher just referred to was
experimenting with the great battery of the Royal Institution of
London. He observed--rather by accident than by design or previous
anticipation--that a strong volume of electricity passing between two
bits of wood charcoal produces tremendous heat, and a light like that
of the sun. It appears, however, that Davy at first regarded the
phenomenon rather in the nature of an interesting display of force
than as a suggestion of the possibility of turning night into day.

For nearly three-quarters of a century the discovery made by Sir
Humphrey lay dormant among