This method dates back to the introduction of the incandescent light,
and, although it has been frequently demonstrated that a small current
of high potential could be employed to work incandescent lamps, the
series system has never been installed on a commercial scale, and is
confined to arc lighting. In the United States the usual pressure for
arc lighting is 2,000 volts, and it is not an uncommon occurrence to
have forty arc lamps in series upon a line over 10 miles in length,
carrying a current of 10 ampères. To economically use this high
pressure for glow lamps in series, they must be of such design as to
enable the whole of the current to be passed through them without
injury. The filament of an ordinary high-resistance glow lamp would
be immediately destroyed, so that low-resistance lamps, having a much
larger sectional area, must be employed. The Bernstein or the Cruto
lamp, which can be made to have a “hot” resistance of about 0·7 ohm,
and requires a current of 9·75 ampères, could be used, and the current
might be economically brought from a great distance. Mr. Bernstein
calculates that it would be possible to operate 6,000 of these 7-volt
lamps from twenty dynamos, each giving a current of 10 ampères at
a potential of 2,000 volts, and still have a margin for loss of
current in the leads. An economical feature of this scheme is the
easy way in which power could be saved when only comparatively few
lights were required; for instance, in the daytime all the circuits
could be looped together and fed by one dynamo, and, as the number of
lights increased, so other machines could be switched in by having
an auxiliary bank of lamps as a resistance. From the central-station
twenty pairs of carefully insulated copper wires, say of No. 6 B. W.
G., would lead to the houses; and, as a good-sized ordinary house takes
on an average twenty lights, the conductor would pass through fifteen
houses before it returned to the station. It is in the house that the
practical difficulty commences, as in this series system the circuit
must never be opened, so that the switches and safety appliances must
be such that, whatever happens, there must remain some path for the
current, otherwise all the lights on that particular circuit would be
extinguished. Mr. Bernstein gives the designation of “short closed” if
the current goes through the switch-lever, and “long closed” if the
current is led through the lamps or other electrical devices.
[Illustration: FIG. 20.]
Fig. 20 is a diagram of the lamps in any building. The street main, M,
enters at the main switch, S, and continues from switch to switch, S¹
S¹, and returns to S before it leaves. It is necessary, to guard
against any possible extinction, to construct all the switches so that
it would be impossible to move the lever without a lamp was lighted;
and, should the lamp give out, an equivalent resistance must be
automatically inserted. These details have been investigated by Mr.
Alexander Bernstein, who has designed a complete system for “series”
lighting, and claims for it special economical advantages. It is,
however, very doubtful if this plan can be recommended for adoption
in private houses; but in public lighting, or in large establishments
where an electrician could be kept to look after the fittings and
the insulation of the conductors, there should be no more danger, in
introducing the high-tension continuous current of 2,000 volts, than
there is at present with the 100-volt alternating current, and the
relative saving in weight of conductors would be an important item.
Installations on this method have been erected at Messrs. Brunner and
Mond’s alkali works, and in several large factories in the United
States where lights had to be distributed over a considerable area; the
system has not, however, come into favour for central-station work.