18 



DISCOVERY 



points along the line to convert the current to con- 

 tinuous current at a pressure suitable for the trains, 

 if the advantages of continuous-current motors, in the 

 power they will develop over a wide range of speed, are 

 to be retained. These sub-stations entail running 

 machinery, and the cost of their equipment, mainten- 

 ance, and operation is a considerable matter to be 

 reckoned with. The use of alternating current trans- 

 mission and converting sub-stations is not limited to 

 heavy railways, but is employed in some of the larger 

 tramway systems such as the London County Council's. 

 At the voltages necessary for the ordinary con- 

 tinuous-current traction motor of the days we are 



supplied to the conductor rails and picked up by the 

 trains. In this way Lots Road runs the District 

 Railway, nearly the whole of the Tube Railways, and 

 a small portion of the London United Tramways. 

 A similar system is used by the Metropolitan, Great 

 Western, London and North Western, London and 

 South Western Railways for their suburban lines, and 

 is employed for many lines of like nature in other 

 countries. 



For long distance and less dense traffic, however, 

 the cost of these converting sub-stations is a very 

 serious item. One way in which the problem has been 

 solved is by using alternating currents throughout, 



Fig. I.— new SINGLE-PH-^SE LOCOMOTIVE FOR THE RH.liTI.^N R.\II.\V.\Y. SWITZERL.-^ND. 



speaking of, an overhead conductor for working heavy 

 trains would have been of unwieldy size, so the insulated 

 " live " conductor rail along the track was used 

 instead. The system was thus arrived at which 

 became practically standard for urban, suburban, 

 and what is sometimes called interurban working, 

 where the traffic is dense enough to justify the cost of 

 numerous sub-stations. In London, for instance, the 

 great power house at Lots Road, Chelsea, supplies 

 alternating (three-phase) current at 11,000 volts to a 

 large number of sub-stations distributed all over the 

 London area, where it is converted to continuous 

 current at about 600 volts, at which pressure it is 



because such currents can be changed in pressure or 

 transformed without running machinery. This enables 

 much higher contact-line pressures to be used with 

 correspondingly lower currents for which overhead 

 conductors are again suitable. The most widely used 

 alternating-current traction system is that known as 

 " single-phase," in which a simple alternating current 

 requires only one overhead wire, and an earth return. 

 The motors for this system were developed later than 

 the continuous-current railway motor and are some- 

 what similar, although they are controlled rather 

 differently, and in some ways are not so satisfactory. 

 The single-phase system is considered by many to be. 



