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._]