Engineering Wonders of the World
Volume III

THE ELECTRIC POWER-STATIONS OF LONDON. 229 Future Supply. the basement. In this way an immense quan- tity of energy may be transmitted by means of a comparatively small wire, and a vast sav- ing effected in the initial outlay on that ex- pensive metal, copper. The periodicity of the current supplied from Deptford is 85 ; that is to say, a “ wave ” of electricity flows back and forth from the gen- erating station through the whole network of mains (about 160 miles) and the wires on the consumer’s premises, 85 times a second. It is fairly safe to prophesy that in years to come the existing power-stations in Lon- don will tend to become merely distributing centres for their vicinity. Huge power-stations, far from the Metropolis, in places where land is cheap, and fuel and water more readily obtained, will probably supply the present sta- tions with high-tension current in bulk. Such a scheme was, to some degree, shadowed forth in a recent proposal of the London County Council; and a beginning of it may be seen in the case of the Central Electric Supply Company, which, from a generating station at Marylebone, supplies additional current to the St. James’s and Pall Mall Electric Light- ing Company and the Westminster Electric Supply Corporation. In this instance a high- tension alternating current (6,000 volts) is conducted to sub-stations in the districts of the two latter companies, at which it is changed to a low-tension direct current for distribution by the existing three-wire system. This affords an example of the combined system of distribution previously mentioned. The conversion at the sub-stations of alter- nating to direct current is accomplished by causing the former to drive an alternating- current motor, the shaft of which is coupled to a direct-current dynamo. Such a com- bination is termed a “ motor-generator.” The question now arises, why should direct current be used in some districts and the alter- nating in others, when the conditions are about the same throughout ? Perhaps the best answer that can be given is that, in the early days of commercial elec- tricity, lighting formed the Alternating t r i • £ i-i and Direct chief business, tor which pur- _ . Currents pose direct and alternating neetjed. currents were equally suitable, apart from the advantages possessed by the latter in regard to transmission. But when the electric motor came to be applied to industry, a stimulus was given to the direct-current sys- tem, as the alternating-current motor had not then been developed on practical lines. Further, it was, and still is, impossible to charge secondary batteries with an alternating current. By the use of a rectifier, however, an alternating current can be changed to a direct current for that purpose. The difficulties connected with alternating- current motors have now been overcome, but a change of system would be attended with inconvenience, so that, although alternat- ing current is, in some cases, supplied by trunk mains to the sub-stations of direct-cur- rent systems, it is converted before its distri- bution through the network. Occasionally— for example, the North and the South Metro- politan Electric Light and Power Companies, which supply a large suburban area—gener- ating plant for both alternating and direct currents is installed at the power-station. It is some twenty years since the public supply of electricity was commenced in Lon- don, and power-stations are still being erected. Many improvements have taken place during this period, and although London is, in the opinion of most people, adequately supplied in this respect, the result is due not to any one general scheme, but to a great number of small schemes carried out in many ways, and owned by various companies and authorities. To describe adequately the manner in which the Metropolis is supplied with electricity would require a more or less detailed account of each of the thirty-fotir areas supplied by the several