ge, as well as to electrical circuits of the approximate
potential to operate them.

Briefly stated, the principle involved in the three-wire system is to
provide main circuits of double the standard potential, so as to operate
standard lamps, or other translating devices, in multiple series of two
to each series; and for the purpose of securing independent, individual
control of each unit, to divide each main circuit into any desired
number of derived circuits of standard potential (properly balanced)
by means of a central compensating conductor which would be normally
neutral, but designed to carry any minor excess of current that might
flow by reason of any temporary unbalancing of either side of the main
circuit.

Reference to the following diagrams will elucidate this principle more
clearly than words alone can do. For the purpose of increased lucidity
we will first show a plain multiple-series system.

In this diagram G<1S> and G<2S> represent two generators, each producing
current at a potential of 110 volts. By connecting them in series this
potential is doubled, thus providing a main circuit (P and N) of 220
volts. The figures marked L represent eight lamps of 110 volts each, in
multiple series of two, in four derived circuits. The arrows indicate
the flow of current. By this method each pair of lamps takes, together,
only the same quantity or volume of current required by a single lamp in
a simple multiple-arc system; and, as the cross-section of a conductor
depends upon the quantity of current carried, such an arrangement as
the above would allow the use of conductors of only one-fourth the
cross-section that would be otherwise required. From the standpoint of
economy of investment such an arrangement would be highly desirable,
but considered commercially it is impracticable because the principle of
independent control of each unit would be lost, as the turning out of a
lamp in any series would mean the extinguishment of its companion also.
By referring to the diagram it will be seen that each series of two
forms one continuous path between the main conductors, and if this path
be broken at any one point current will immediately cease to flow in
that particular series.

Edison, by his invention of the three-wire system, overcame this
difficulty entirely, and at the same time conserved approximately, the
saving of copper, as will be apparent from the following illustration of
that system, in its simplest f