cial value.

Edison then turned his attention in other directions. In his earliest
consideration of the problem of subdividing the electric current, he had
decided that the only possible solution lay in the employment of a lamp
whose incandescing body should have a high resistance combined with a
small radiating surface, and be capable of being used in what is called
"multiple arc," so that each unit, or lamp, could be turned on or off
without interfering with any other unit or lamp. No other arrangement
could possibly be considered as commercially practicable.

The full significance of the three last preceding sentences will not be
obvious to laymen, as undoubtedly many of the readers of this book may
be; and now being on the threshold of the series of Edison's experiments
that led up to the basic invention, we interpolate a brief explanation,
in order that the reader may comprehend the logical reasoning and work
that in this case produced such far-reaching results.

If we consider a simple circuit in which a current is flowing, and
include in the circuit a carbon horseshoe-like conductor which it is
desired to bring to incandescence by the heat generated by the current
passing through it, it is first evident that the resistance offered to
the current by the wires themselves must be less than that offered by
the burner, because, otherwise current would be wasted as heat in the
conducting wires. At the very foundation of the electric-lighting art is
the essentially commercial consideration that one cannot spend very much
for conductors, and Edison determined that, in order to use wires of a
practicable size, the voltage of the current (i.e., its pressure or
the characteristic that overcomes resistance to its flow) should be one
hundred and ten volts, which since its adoption has been the standard.
To use a lower voltage or pressure, while making the solution of the
lighting problem a simple one as we shall see, would make it necessary
to increase the size of the conducting wires to a prohibitive extent.
To increase the voltage or pressure materially, while permitting
some saving in the cost of conductors, would enormously increase the
difficulties of making a sufficiently high resistance conductor to
secure light by incandescence. This apparently remote consideration
--weight of copper used--was really the commercial key to the problem,
just as the incandescent burner was the scientific key to that problem.
Before Edi