PROTECTION FROM LIGHTNING. 205 It will be seen that the total saving in favour of the more expensive silicon bronze wire is very considerable. It should also be remembered that this wire is hardly at all affected by the atmosphere, and that it retains nearly its full market value after the equivalent iron wire has been rendered valueless by rust. Protection from Lightning. Overhead lines, whether used for electric lighting or Fig. 78. To Earth transmission of energy, are exposed to the effects of lightning, which may not only destroy the line, but also the dynamos or motors at either end. To protect the plant, various methods are in use, all of which are more or less modifications of the lightning protectors used in telegraphy, and which are based on the principle that a lightning discharge can leap a small break in a circuit to earth, which to the working current is impassable. Fig. 78 shows a lightning protector for the line. The vertical point acts as an ordinary lightning protector, so as to minimize the risk of the line being struck. Should this nevertheless happen at some place between two protectors, then the current will travel along the wire, and leap across to the horizontal point, and thus be conducted to Fig. 79. C H= E earth before it can reach the machinery at either end of the line. Another arrangement intended for the same purpose is that invented by Professor Thomson in connection with his system of arc lighting over long distances. It is, of course, equally applicable to long lines used for the transmission of energy. This protector, Fig. 79, permits a discharge to earth from both the posi tive and negative line, and it moreover automatically PROTECTION FROM LIGHTNING. 207 ruptures any short circuit of the lines which may be thus started by the lightning current. It must be remembered that the electro-motive force employed in the ThomsonHouston system is very high (up to 2,000 volts), and that if an arc is formed between the metal parts of the protector, this electro-motive force would be high enough to sustain it after the lightning stroke has passed, and thus not only damage the protector, but possibly also burn up the dynamo. To prevent this a device is employed which automatically ruptures the arc. In Fig. 791 G is the dynamo working the line, C; L and L', are plates of metal in connection with the line, and e and e1 are similar plates in connection with earth at E, E1. There is a small interval between e and L, and between e' and L, which, when a lightning discharge falls on the line, is easily leaped by it. The stroke is thus carried to earth. To rupture the arc formed, a magnet, M, Fig. 80, is employed. The plates L and E approach closely only at their lowest parts, and above are spread out as shown. The effect of the magnet, whose poles are flattened out as shown, is to repel upward the arc that may be formed between the plates L and E. The arc at once rises to the wider space and is thus broken. Underground Lines. A large number of systems of underground cables have been either proposed or tried, but as yet it cannot be said that anything like finality has been reached. The mains have been, and are still the most serious difficulty of electrical distribution. Edison was one of the first to 1 The Author is indebted to the Editor of "The Electrical Review" for the use of this illustration and of Fig. 80. grapple with the problem, and may be said to have found a solution which, if not perfect, at least has the merit of working. He originated the system of placing two halfround conductors into an iron pipe, the space between the conductors, and between them and the pipe being filled by a bituminous compound, which was poured in when liquefied by heat. The main was made in 20feet lengths, the copper strips protruding at each end for convenience of jointing. The joints were made by soldering, which proved a very troublesome operation, because the thick copper strips carried the heat away almost as UNDERGROUND LINES. 209 fast as it was applied to the joint. To minimize this trouble it would have been advantageous to employ longer tubes, but that was found impossible, as the streets of New York, like those of any large town, are so cut up by gas, water, and drain-pipes, that no straight line of any length can be obtained. A short coupling-tube was placed over each joint connecting the iron pipes. Very soon after the installation was started troubles arose. The unequal nature of the ground, and the strains arising from the heavy traffic on the streets caused the pipes to be bent or broken, the conductors were thereby strained and worked through the bituminous compound until they came in contact and formed a short circuit, and the pipes were often accidentally damaged by the tools of workmen engaged upon some gas, water, or sewer work. The light wrought-iron piping at first employed was by degrees exchanged for strong steam-piping which could not easily be broken through by a pickaxe, and greater flexibility was given to the whole system by replacing the rigid couplings by ball-and-socket joints which permitted the mains to follow more or less any subsidence in the ground which might take place. The copper strips before being inserted in the pipe, were each taped singly, then served spirally with cord, laid together with their flat sides, and again wound spirally with cord. This prevented their coming in contact with each other, or with the sides of the pipe, even if the insulating compound should give way. Where a bend in the main is necessary, cast-iron elbows, as shown in Fig. 81, are introduced. It is impossible to speak of underground conductors without making a digression to explain the so-called "three-wire system." It has already been pointed out that electric distribution, to be perfect, must insure the Р |