DISCOVERY 



17 



point at which the animal had emerged from the wood. 

 I followed its footmarks until I came upon it squatting 

 in a nice cosy ' seat ' tmder a bush. There were 

 plenty of good burrows in the field it could have 

 taken to for safety, supposing it had been driven out of 

 the wood bj- a pursuing enemy, but the fact that it 

 had entered the field in quite an unperturbed state of 

 mind, and had not attempted to go near any hole or 

 burrow, entire!}' mystified me as to why it should be 

 going to rest at such a peculiar hour of the day." — 

 Richard Kearton, F.Z.S., in At Home with Wild 

 Xatiire. (CasseU & Co., 7s. 6d.) 



Railway Electrification 



By S. R. Roget, M.A., A.M.I.E.E. 



The " electrification " of railways, or as some engineers 

 prefer to call it, the conversion of railways to electric 

 traction, is a matter which is receiving more and 

 more attention in all parts of the world. Its progress 

 was considerably retarded by the war and the con- 

 ditions induced thereby, but there are, at the same 

 time, in these conditions themselves features which 

 increase its advantages and may render the adoption 

 of electric traction on the railways of the world ulti- 

 mately more universal, even if somewhat delayed. 

 Before discussing this present-day aspect of the 

 problem, however, it may be of interest to look back a 

 httle into the nature of the technical problems which 

 have been faced, the systems which have been developed 

 to meet them, and the directions in which practice 

 is pointing to the future. 



Broadly speaking, electric traction comprises any 

 system in which an electric motor propels a vehicle by 

 driving its wheels, or otherwise exerts a force to move 

 it. The current which provides energy for the purpose 

 may be supplied from a stationary source or from one 

 moving with the vehicle. On electric railways the 

 source of energy is usually a fixed generating station 

 whose power is provided by steam turbines, engines, 

 or other prime movers, or by water-power, the motors 

 being carried on the train ; for it is not practicable to 

 drive trains by storage batteries, nor is haulage by 

 fixed electrically driven machinery suitable except 

 for such special cases as short mountain Unes carrying 

 comparatively light loads. 



Tram\vays 



The ver}' earliest examples of electric traction were 



experimental, and out of them grew the familiar 



electric tramway. In the typical electric tramway, 



two electric motors on each car, mounted on the 



imderframes or bogies, drive the axles through gearing, 

 and continuous current (i.e. current always flowing 

 in the same direction) is supplied from an overhead 

 wire by a " trolley " or other contact device, and 

 returns to the generating station through the running 

 rails and the earth. The system is satisfactory from 

 a technical point of view. One of the principal troubles 

 experienced w'as in connection with the return current 

 through the earth. As the raUs were not insulated, 

 the return current sometimes wandered through the 

 ground and, passing through any buried pipes or other 

 metal-work on its way, caused serious corrosion by 

 electro-chemical action. Means were, however, found 

 to keep the return current within reasonable bounds 

 through limiting the pressure which could exist between 

 rail and earth by introducing a few volts in the opposite 

 direction by an auxiUarj' apparatus know'n as a 

 " booster " in the station. In very exceptional cases 

 the earth return has been abandoned and two over- 

 head wires employed, notably in the case of certain 

 lines near Greenwich Observatory where the stray 

 return currents would be liable to interfere with 

 observations of terrestrial magnetism. 



Development of Electric Railways 



The earlier and many of the present electric railways, 

 whether formerly worked by steam or not, are in their 

 essentials practically enlarged electric tramways. 

 Naturally this increase in scale introduced various 

 special conditions which brought about modifications 

 and developments. It is a far cry from the single 

 car with its comparatively simple controller, by which 

 the driver regulated the supply of current to the 

 motors, to the large passenger motor-coach, any 

 number of which can be joined together, with or without 

 other motorless coaches, to form a train. In some 

 cases, particularly where old steam roUing stock or 

 through trains running on to steam-worked lines have 

 to be handled, or in goods traffic, or on long-distance 

 lines worked at high pressures, electric locomotives 

 upon which all the electrical apparatus is concentrated 

 are preferable, and here the departure from tramway 

 practice is most complete. 



Various Systems of Current Supply to the 

 Trains 



The increased powers and distances dealt with also 

 made it advisable to take advantage of the economy 

 in transmission presented by " alternating " currents 

 (i.e. currents which change their direction many times 

 a second) owing to the ease with which such can be 

 generated at, or " transformed " up to, high pressures ; 

 for the higher the pressure, the smaller is the current 

 required to convey a given quantity of energj'. This 

 system, however, requires a number of sub-stations at 



