M M′ are the flow and return mains, the dynamos bridging them across at one end. If the mains were very long, those near to the dynamos would be exhausting the supply, and the lamps at the remote end would not get the full pressure. A system of feeders has been devised so that each lamp, no matter where it may be, shall have approximately the full 110 volts working through it. [Fig. 17] shows a long circuit consisting of two branch mains bridged by a large number of lamps, l l, and D D are the dynamos at the central-station. Series of feeders, f f′, have to be taken from the dynamo mains and fed direct into the branch mains at various points, d d′, b b′, c c′, in order to distribute the electrical pressure equally.
The Three-Wire System.
The ordinary parallel system is undoubtedly suitable for small installations; but when the area to be lighted is extensive, it is impossible to proportion the mains, with a view to economy in the cost of copper, without sacrificing energy wasted in heating the conductors.
In Figs. 16, 17, the lamps are shown in simple parallel; but if two dynamos are connected together, and a main wire is run from each of their two extreme terminals and a third wire from the branch connecting the two machines, we have what is known as the three-wire system, which was invented by Edison in America, and Hopkinson in England, almost simultaneously. Although by using the third wire there is a saving in copper over the parallel plan, the maximum gain is not more than 25 per cent., under the best conditions; when the district to be illuminated is not more than 400 to 600 yards from the central-station, the three-wire system answers well, but as soon as this distance is exceeded the cost of the mains begins to mount up at a most alarming rate. Although there are many Edison installations in the United States on this system and a few on the Continent, it has only been used here in a few instances for factory lighting.
The Edison System at Milan.
The Santa Radegonda station at Milan is at the present moment the second largest Edison station in Europe. The building, which was formerly a theatre, is well adapted for the work required; the dynamos and engines are fixed in a deep basement, while the boilers are a few feet above the street level, the upper floors being used as stores and testing-rooms. The dynamos, eight in number, are of the old Edison type, with horizontal magnets; seven of these machines are connected to the feeders which supply the mains, and these cover the district to be lighted on the Edison network system. The motive power is furnished by six Armington-Sims, and two Porter-Allen engines, each connected direct to the armature of a dynamo, the speed being maintained at the uniform rate of 350 revolutions per minute, except in the case of the spare engine and dynamo, which is kept turning slowly, ready to be switched on should occasion demand. The starting or cutting-out of circuit of these large machines requires some care. In the first place, to start, it is necessary to insert resistance into the shunt circuit of the dynamo, which is done by a switch; but to throw 150 horse-power into the main circuit would be dangerous to the lamps, so that the current is first sent into a bank of one thousand lamps used as a resistance, and these are cut out step by step; similar care is taken when a machine is stopped. To control the electro-motive force, which varies greatly from time to time, hand regulation is used during the day, with the help of the Edison tell-tale, consisting of two lamps, a red and white one, which light up when the current is high or low; but when the night service comes on, as it may happen that two thousand lamps may be turned out at once, an attendant has to carefully watch the electric regulator, and be ready to insert resistance into the field-magnet circuits by moving a wheel connected by a shaft and bevel-gear to a system of commutators. The principal difficulty to be overcome, in an installation where the current is distributed over a large area, is the regulation of the electro-motive force at the various points, as at Milan; there are no return galvanometer wires, which are now used in both the two and the three-wire Edison systems in the United States. The plan devised by the company’s electrician at Milan is very ingenious, and enables the pressure at the ends of the various feeders to be kept practically the same, although they are of different lengths and sectional area. In the first place, resistance was added to each feeder to equalise the resistance in each conductor; and, in order to provide for the varying amount of current the feeder has to supply, a peculiar form of commutator, having a guillotine-shaped contact-piece, was inserted in the circuit. By moving this, suitable resistance is inserted or cut out, and the attendant, having a series of numbers, has only to set this instrument to the number shown by the ampère meter. By far the largest amount of current is drawn off for the lighting of the Scala Theatre, the stage-lighting alone taking more than one thousand lights: if these were all turned on suddenly, the other lights in the district would be dimmed; to obviate this, auxiliary feeders have been run, which are used only when any great increase is expected; commutators similar to those referred to above also regulate these feeders without any special attention. The pressure at any point in the system is by this means easily controlled, and affords an illustration of what is perhaps not the most economical, but is found to be the most practicable, way of maintaining a constant potential in a district where the amount of output of current is suddenly doubled. [Fig. 18] is a plan of the network system of conductors laid through a large portion of the city; the conductors are in outward appearance similar to gas-pipes, the current passing through semicircular bars of copper, embedded both for the flow and return in the same iron tube, which is laid underground in a shallow trench. The house-supply is drawn from the mains, and these are connected to the feeders by means of ordinary junction-boxes, which each contain a fusible cut-out. The bridge-boxes allow of expansion of the line, and have connections for testing purposes. The insulation is extremely good, mainly on account of the favourable nature of the ground, which is chiefly gravel; no trouble has been experienced with leakage, nor has the service ever been interrupted. The cut-outs are of an improved Edison form, but have the disadvantage attending all lead plugs where the current is great, in that, to guard against accidental melting due to the heating effect of the current, the sectional area of the lead has to be much larger than would be otherwise necessary. In fact, these cut-outs will protect the cable against a bad short circuit, but nothing else.
Fig. 18.
In addition to the glow lamps, eighty arc lamps are worked in derivation, two in series; most of these lamps require 45 volts, to which 10 per cent. of idle resistance is added, constituting a total loss of current which is extremely low for a combined arc and incandescent system of lighting. The service commenced in 1882 with a little over one hundred lamps, and at present there are over ten thousand glow lamps, and two hundred arc lamps are in use. At first the new enterprise had to struggle against very great difficulties; not only the technical difficulties of distribution by means of a network of feeders and mains had to be overcome, but also those arising from the prejudices of consumers and the competition of the gas company, who tried to deter consumers from introducing electric light into their houses. One of these means consisted in offering to the private consumers, resident in the district which was threatened by competition with electricity, an agreement by which the gas company bound itself to supply gas at 5s. 8½d. per 1000 cubic feet, instead of 7s. 7d. as charged hitherto; and even now those inside the “charmed circle” of the electric light conductors get their gas cheaper than the public outside. One of the reasons which accelerated the adoption of electric light was the introduction of the Edison meter, in consequence of which consumers could be charged exactly for the amount of light they had received, and were relieved from paying a lump sum according to the number of lamps fixed, which was customary in the early days of the company. The prices at which the company now provides light, at all hours of the day and night, are as under:—