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 connect- ing 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.