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:— 10-candle lamps 2·5 pf., about 0·29 _d._ per hour. 16- ” 4·0 ” 0·48 ” 32- ” 8·0 ” 0·96 ” 50- ” 12·5 ” 1·50 ” 100- ” 25 ” 3·00 ” In addition to this an installation fee of 6_s._ per lamp is charged, which includes one lamp. Meters are charged as follows:— _£ s. d._ 10- to 16-candle-power 0 16 0 per annum. 25- ” ” 1 0 0 ” 50- ” ” 1 10 0 ” 100- ” ” 2 0 0 ” A discount is allowed off this meter charge, varying with the number of hours the light is used in the year. The cost of gas is about 4_s._ 9_d._ per 1,000 cubic feet, so the electric light is slightly the dearer illuminant. The Aron meter, Fig. 19, is usually employed as the recorder of the electricity consumed. It consists of two pendulums, controlling two distinct clockwork gears. One oscillates at a regular speed, but the other has a permanent magnet, instead of a weight, and is variable in speed. The entire current passes through the solenoid, which is underneath the pendulum, with the magnet; the difference in speed between the standard and variable clocks is given in direct ampère-hours by a counter-gearing similar to the index of a gas-meter. An electro-magnet starts each pendulum when the current begins to flow, and immediately it ceases, two detents come into operation and hold the pendulums stationary. [Illustration: FIG. 19.] CLASS III. THE SERIES SYSTEM OF DISTRIBUTION.