It will be found, on examining Appendix II., that in European stations
by far the larger number of lamps are maintained from installations
employing the Edison system; the Ferranti plan of using transformers
comes next, closely followed by Goulard and Zippernowsky; the
distribution with secondary batteries follows, and the high-tension
multiple series comes last.
[Illustration: FIG. 16.]
The Edison system has frequently been discussed, in connection with
small installations, but in magnitude the stations in Berlin and in
Milan exceed anything that has been started here with continuous
current.
Before describing the central electric light station at the former
city, it may be well to recall to mind that the Edison plan is the
combination of a number of machines which pump electricity into a
network of feeders, mains, and conductors, the lamps being placed in
parallel circuit, as shown at L _l_, Fig. 16, and maintained at a
constant potential of 110 volts.
[Illustration: FIG. 17.]
M M′ are the flow and return mains, the dynamos bridging them across at
one end. If the mains were very long, those near to the dynamos would
be exhausting the supply, and the lamps at the remote end would not get
the full pressure. A system of feeders has been devised so that each
lamp, no matter where it may be, shall have approximately the full 110
volts working through it. Fig. 17 shows a long circuit consisting of
two branch mains bridged by a large number of lamps, _l l_, and D D are
the dynamos at the central-station. Series of feeders, _f f′_, have to
be taken from the dynamo mains and fed direct into the branch mains
at various points, _d d′_, _b b′_, _c c′_, in order to distribute the
electrical pressure equally.