One central generating station has been provided for the whole town,
from which at present four distinct circuits have been laid, each fed
by a separate dynamo. The street lamps are connected up in “multiple
series,” that is to say, in groups placed in series on the circuit, the
lamps in each group being connected up in parallel.
Fig. 21 shows the arrangement diagrammatically. Each group consists
of eight lamps in parallel; at present three of the circuits have
twenty-four groups in series, and the fourth circuit has twenty-three
groups in series, giving a total of ninety-five groups, comprising 760
lamps, of which 731 are public lamps and 29 are used at the central
station. To meet the risk of interruption in any circuit through
the failure of individual lamps, an automatic switch is arranged so
as to put in a reserve lamp, in the event of a whole group being
interrupted. Another self-acting device will short circuit the whole
group, so that the other groups in the circuit will be unaffected. The
automatic lamp-switch is contained, together with the reserve lamp,
in the lantern, and the automatic group cut-out consists simply of an
electro-magnet with a coil of high-resistance connected up in parallel
with the group of lamps it protects. These appliances have been found
to work well. The main conductors are formed of insulated single
copper wire, 4·6 millimetres in diameter; they are carried overhead on
porcelain insulators, fixed to telegraph posts or to wooden arms let
into the walls of houses; the resistance of this conductor is about 1·1
ohm per kilometre. The glow lamps are placed in reflectors at an angle
of about 45° from the vertical, and are carried on brackets either
fixed to the walls or on special cast-iron posts. Fig. 22 shows the
details of street bracket and reflector with automatic lamp-switch and
lamps in place. The brackets are for the most part fixed to the walls
of houses or to painted wooden posts.
[Illustration: FIG. 22.