“The chief points of the system is the use of a generator in a central
position, from one pole of which insulated conductors or mains are led
to the several points where the electric energy is to be utilised,
being branched and sub-branched as much as required, and thence back
to the other pole of the generator by an uninsulated conductor, such
as the gas or water pipes. At certain points, storage or secondary
batteries are set up in connection, on one hand, with the mains,
sub-mains, and branches, as the exigencies of the case may require,
and, on the other, with the return conductor.”
[4] Hedges on the Supply of Electricity by Local Authorities.
Proceedings of Association of Municipal and Sanitary Engineers and
Surveyors, vol. ix. (1882-83), p. 159.
“The combination of generators, circuit and storage batteries is such,
that when the current from the generators falls below the demands made
on it from the various outlets to the mains at which its energy is
utilised, the deficiency is made up from the storage batteries, which
act in unison to supply the requisite quantity of energy. On the other
hand, when the current from the generator exceeds in point of quantity
the demands upon it at the various outlets, the excess goes to charge
the storage batteries and to create a reserve to be called upon in case
of need.”
The objection to the system which prevented it being put in practical
operation was the use of the earth as a return conductor. Besides the
great danger of short circuit, the gas and the water pipes, which
are so thickly laid in most cities, would conduct the current and
interrupt telegraphic and telephonic communication. The experiment of
using storage batteries as reservoirs, from which a constant supply of
electricity could be drawn as required, was tried on a considerable
scale at Colchester, where a large installation was started in 1884,
secondary batteries being placed in favourable positions, and charged
by a high-tension current. The plan adopted is shown by Fig. 24.
[Illustration: FIG. 24.
A is a meter in charging circuit; B, the batteries or accumulators; L,
lamps in parallel on low-pressure service main.]
The dynamos were two of the Brush type, each dynamo giving a current
of 9·5 ampères, with an electro-motive force of 1,800 volts, when
rotated at a speed of 700 revolutions per minute. They were driven by
a semi-portable engine indicating 90 horse-power. The dynamos were
coupled in parallel circuit for quantity, and excited by a small
machine giving 10 ampères. The current was led some distance by a
seven-strand 19 B. W. G. cable to the batteries, which were charged in
series, the 60-volt lamps being placed in parallel on separate mains
connected to the batteries. The danger of introducing a high-tension
current of 1,800 volts into the houses was obviated by a rocking-switch
worked automatically, so as to throw the batteries out of the charging
circuit. The operation was accomplished by means of a master cell M,
C, Fig. 24, similar to the others, but fitted with an arrangement
to collect the gas evolved, which extended a diaphragm attached to
a make-and-break arrangement which worked the rocking-switch. The
Colchester installation did not turn out commercially successful, and
has been abandoned; but the experiment has been valuable, and there
is little doubt that, with simplification of details, a high-tension
charging current could be led from a dynamo fixed in any convenient
site where power is available; also in very crowded districts the
batteries could be placed in cellars and be drawn from as reservoirs,
so as to furnish a constant supply of electricity.
[Illustration: FIG. 25. _To face page 69._]
The Kensington Court installation has been previously quoted as an
example of what promises to be one of the most successful methods
of distributing a constant supply of electricity through a large
area, a description of the station may therefore be interesting. The
accompanying elevation, Fig. 25,[5] shows the unpretending design
of the building, and the very compact arrangement of the generating
machinery and batteries. When the illustration was made the plant
consisted of one Willan’s single-crank triple-expansion engine in
combination with a Crompton dynamo provided with vertical inverted
single magnets, the output being 250 ampères at 140 volts when running
at 500 revolutions per minute, the steam pressure being 160 lbs. on the
square inch. A complete duplicate plant has already been installed, and
three more sets of engines and dynamos are shortly to be erected. The
draught from the boiler is led downwards by an underground flue, with
the object of economising the very limited space as much as possible.
As a rule, the dynamo and accumulators are used in parallel, the
current enters and leaves the regulating cells by the same contact,
in other words, there is only one switch which serves for charging
and discharging the batteries. This switch has nine contacts, so as
to give nine degrees of regulation of the light; when the dynamo and
accumulators are working together, the lights are parallel with either
41, 42, or 43 cells, according to the amount of charge in the cells and
current required, while, when the dynamo is out of circuit, the lights
are worked off, 50, 51, 52, or 53 cells. The current passes through the
usual measuring instruments, and each main conductor is protected by
safety fuses mounted in a Hedges duplex cut-out. The accumulators are
of the Planté type, but instead of being plain lead are sawn out of
ingots which are cast porous on the Howell process. Each cell contains
35 plates, 8 in. × 8 in., and, as each plate when fully formed is said
to be capable of yielding five ampère-hours per pound of lead, the cell
has about 600 ampère-hours total capacity. In the event of a serious
breakdown the whole of the work would fall on the accumulators, which
could furnish a steady current for perhaps an hour or more; and herein
lies the novelty of the arrangement. For the first time we have an
accumulator put in not only as a fly-wheel to the whole system and
to give the advantage of supplying current throughout the day and
the small hours when the engine is not running, but also to act as
an actual reserve. The routine is as follows:—the dynamo will start
charging the accumulators a few hours before dusk; for a short time
after lighting hours commence, the dynamo alone will supply sufficient
current, but later on the demand will gain on the dynamo, and a certain
portion of the discharge will be from the accumulators. At eleven
o’clock at night the engine will be stopped and the accumulators will
alone supply the demand for the rest of the night. In the small area
occupied by the station there is ample room for a plant of six times
the present capacity, and it is intended to erect sub-distributing
stations at points at the outskirts of the district where accumulators
to act as transformers will be fixed, which will be charged by a
special main with a current of 500 volts, the outgoing wires from the
sub-station taking electricity at the usual E.M.F pressure for
incandescent lamps in houses of 100 volts.
[5] From _Industries_.
Thirteen candle lamps are used in the district, having been found to be
more convenient than 16 or 20 candle-power, the 13 candle is obtained
for 36 watts, or 2·75 watts per candle. The price charged to consumers
is 8_d._ per Board of Trade unit, or equivalent to gas at about 4_s._
7_d._ the 1,000 cubic feet. Meters on the Aron plan, Fig. 17, are
used, a card being supplied on which the readings are entered exactly
similar to the method adopted with gas. The service mains terminate at
the meter, where the company fix for their own purposes a double pole
switch of the author’s design, Fig. 26, which enables both wires to
be disconnected, a spring shut-off, marked S S, prevents the
switch being left partly on.