It is evident that in so extensive a system of lighting a short circuit now and then between the lamp wires and the earth cannot altogether be avoided. Many of the lamps have been fitted to existing gas fittings, and are beyond the daily supervision of the company’s officers; the faulty place is often not easily accessible, so the first step taken is to discover on which of the two circuits the trouble has occurred. This is done at the station by joining two 16-candle lamps in series across the main conductors and the point of junction between the two lamps is connected to earth by a stout wire. As long as both circuits (positive and negative) are perfectly insulated from earth no current flows through this middle wire, and both lamps remain hardly incandescent; but, if one of the circuits should be in connection with the earth, the lamp which is joined on the other circuit will brighten up, because the potential of the middle wire and that of the faulty circuit are both zero, and consequently the lamp between the middle wire and the sound circuit receives the full pressure of 110 volts. To localise the fault, contact is made between the earth and the sound circuit by means of a fusible plug of known melting point, say for a thirty-lamp supply. If the fault is on a portion of the external circuit, supplying less than thirty lamps, its fusible plug will melt as soon as the sound main is put to earth. If, however, the fault is on a portion supplying more than thirty lamps, the fusible plug which has been inserted at the station between the sound main and the earth will melt instead. A series of fusible plugs are thus tried, increasing in melting capacity until one is found that does not go: in this case, the other plug on the faulty portion has melted, and the consumer’s lamps on that branch are extinguished; the position of the fault is thus localised, and the company proceed to remedy the defect without interfering in the slightest degree with the rest of their system.
The Edison system is also employed at Berlin, in fact the Deutscher Edison Gesellschaft have at the present time a monopoly of the supply of the city from three large central-stations, each of which serves the area in their immediate neighbourhood. The mains differ from those used at Milan in that stranded highly insulated cables, protected with steel wire on the outside, are laid under the pavement in every street throughout the district. With the exception of the Leipziger strasse and Unter den Linden, which are lit with arc lamps suspended from chains running between cast-iron poles 24 ft. high, about 100 to 250 ft. apart, gas is used for the street lighting, and electricity for the interior illumination of many public buildings and private houses; there are also a good many arc lights outside the shops and restaurants. The mains are on the Edison network system, the area of copper being such, that when all the lamps are on there is a loss of energy of 25 per cent.; but this does not occur on an average for more than half an hour a day. No sole concession is given to the company, who simply have the right to take up the pavement and cross streets, and for this permission they are bound to furnish any consumer in the district with a constant supply of electricity at the following charges:—