CONTINUOUS-CURRENT DYNAMOS. 311 



•of chlorine salts by electrolysis : each of these has an out- 

 put of 1 15 volts and 1200 amperes and is a six-pole machine 

 with drum armature coupled directly to the horizontal shaft 

 of a 220 horsepower turbine. In considering the extent 

 to which the Sayers winding is likely to be adopted in the 

 future, perhaps the most serious question which the dynamo 

 designer has to face is that of the cross armature reaction 

 in large machines. The air-space being small, the loops 

 lying under the pole-pieces have a proportionately great 

 cross-magnetising effect : the induction at the trailing pole- 

 tip is thereby forced up to a high figure, and, if care be 

 not exercised in the design, the iron may become saturated 

 either at the trailing pole-tip or in the teeth of the armature 

 core under it, so that the total number of lines is reduced. 

 Hence in the dynamos which have so far been built on 

 the Sayers principle, a double horseshoe field has been 

 usually adopted, which enables each pole-piece to be split 

 in half and an appreciable air-gap to be interposed in the 

 cross-magnetising circuit, without interfering with the true 

 path of the lines into the armature core. 



Finally we come to the field of practice, and it is here 

 that the advance of the last few years is most marked. As 

 the use of the continuous current for lighting, transmission 

 of power, traction, and electrolysis has steadily increased, 

 so also have the dynamos for supplying it grown vastly not 

 only in number but also in size. Ten years ago, the 

 dynamos in general use were of 10 to 50 horsepower, 

 but now a unit of 200 horsepower is common in British 

 central electric light stations, while on the Continent 500 

 horsepower dynamos are frequently met w T ith, and in several 

 cases monsters of even 1000 or more horsepower are success- 

 fully employed. Their forerunners are to be found in the 

 Edison "steam dynamo" nicknamed "Jumbo," of which 

 the output was 900 amperes at a pressure of 105 volts, and 

 the "Colossus" Brush machine, for which an output was 

 claimed of 3200 amperes and 80 volts at 405 revolutions 

 per minute. The former is worthy of especial note from 

 its anticipation of modern central-station work in its being 

 directly coupled to the crankshaft of a horizontal steam- 



