The ordinary parallel system is undoubtedly suitable for small
installations; but when the area to be lighted is extensive, it is
impossible to proportion the mains, with a view to economy in the cost
of copper, without sacrificing energy wasted in heating the conductors.
In Figs. 16, 17, the lamps are shown in simple parallel; but if two
dynamos are connected together, and a main wire is run from each of
their two extreme terminals and a third wire from the branch connecting
the two machines, we have what is known as the three-wire system,
which was invented by Edison in America, and Hopkinson in England,
almost simultaneously. Although by using the third wire there is a
saving in copper over the parallel plan, the maximum gain is not more
than 25 per cent., under the best conditions; when the district to be
illuminated is not more than 400 to 600 yards from the central-station,
the three-wire system answers well, but as soon as this distance is
exceeded the cost of the mains begins to mount up at a most alarming
rate. Although there are many Edison installations in the United States
on this system and a few on the Continent, it has only been used here
in a few instances for factory lighting.