would flow through the negative, N, back to the source of energy. Thus
it will be seen that a three-wire system, considered as a whole, is
elastic in that it may operate as one when in balance and as two when
unbalanced, but in either event giving independent control of each unit.

For simplicity of illustration a limited number of circuits, shown in
Fig. 2, has been employed. In practice, however, where great numbers
of lamps are in use (as, for instance, in New York City, where about
7,000,000 lamps are operated from various central stations), there is
constantly occurring more or less change in the balance of many circuits
extending over considerable distances, but of course there is a net
result which is always on one side of the system or the other for the
time being, and this is met by proper adjustment at the appropriate
generator in the station.

In order to make the explanation complete, there is presented another
diagram showing a three-wire system unbalanced:

The reference figures are used as before, but in this case the vertical
lines represent branches taken from the main conductors into buildings
or other spaces to be lighted, and the loops between these branch wires
represent lamps in operation. It will be seen from this sketch that
there are ten lamps on the positive side and twelve on the negative
side. Hence, the net result is an excess of current equal to that
required by two lamps flowing through the central or compensating
conductor, which is now acting as positive to generator G<2S> The arrows
show the assumed direction of flow of current throughout the system,
and the small figures at the arrow-heads the volume of that current
expressed in the number of lamps which it supplies.

The commercial value of this invention may be appreciated from the fact
that by the application of its principles there is effected a saving
of 62 1/2 per cent. of the amount of copper over that which would
be required for conductors in any previously devised two-wire system
carrying the same load. This arises from the fact that by the doubling
of potential the two outside mains are reduced to one-quarter the
cross-section otherwise necessary. A saving of 75 per cent. would thus
be assured, but the addition of a third, or compensating, conductor of
the same cross-section as one of the outside mains reduces the total
saving to 62 1/2 per cent.

The three-wire system is in universal use throughout the world at the
pre