ations of loss in the feeders would not give
rise to similar fluctuations in any lamp circuit. The feeders might be
operated at any desired percentage of loss that would realize economy in
copper, so long as they delivered current to the main conductors at the
potential represented by the average voltage of the lamps.

Thus the feeders could be made comparatively small in cross-section. It
will be at once appreciated that, inasmuch as the mains required to be
laid ONLY along the blocks to be lighted, and were not required to be
run all the way to the central station (which might be half a mile or
more away), the saving of copper by Edison's feeder system was enormous.
Indeed, the comparative calculation of Mr. Jenks, above referred to,
shows that to operate the same number of lights in the same extended
area of territory, the feeder system would require only 128,739 pounds
of copper, which, at the then price of 25 cents per pound, would cost
only $39,185, or A SAVING of $168,627.50 for copper in this very small
district of only nine blocks.

An additional illustration, appealing to the eye, is presented in the
following sketch, in which the comparative masses of copper of the tree
and feeder systems for carrying the same current are shown side by side:

XII. THE THREE-WIRE SYSTEM

THIS invention is covered by United States Patent No. 274,290, issued to
Edison on March 20, 1883. The object of the invention was to provide
for increased economy in the quantity of copper employed for the main
conductors in electric light and power installations of considerable
extent at the same time preserving separate and independent control
of each lamp, motor, or other translating device, upon any one of the
various distribution circuits.

Immediately prior to this invention the highest state of the art of
electrical distribution was represented by Edison's feeder system, which
has already been described as a straight parallel or multiple-arc
system wherein economy of copper was obtained by using separate sets
of conductors--minus load--feeding current at standard potential or
electrical pressure into the mains at centres of distribution.

It should be borne in mind that the incandescent lamp which was accepted
at the time as a standard (and has so remained to the present day) was
a lamp of 110 volts or thereabouts. In using the word "standard,"
therefore, it is intended that the same shall apply to lamps of about
that volta