CONTINUOUS-CURRENT DYNAMOS. 149 



This brings us to the great question of multi-polar 

 versus bi-polar dynamos for large outputs. Given an arma- 

 ture core of a certain size, its output, so far as regards volt- 

 age and the current that may be obtained from it without 

 overheating, is, roughly speaking, independent of the number 

 of poles. As, however, we increase the number of poles, 

 the cross ampere-turns acting on each pole are reduced in 

 proportion : if therefore the number of poles be raised from 

 two to four the length of air-gap required to prevent sparking 

 may be reduced to half of that which would be required in 

 the bi-polar machine. 1 Herein lies the advantage of the 

 multi-polar type, but the question when the output is too 

 large to be successfully obtained from a two-pole dynamo 

 cannot be said to have been finally decided as yet. This 

 point will be again alluded to later, and the fact is merely 

 noted here that multi-polar forms have found much more 

 favour on the Continent than in Britain. 



The concluding remarks of the present article will be 

 devoted to the efficiency of modern continuous-current 

 machines. Viewed as a converter of energy from one form 

 into another, the dynamo must certainly be pronounced the 

 most wonderful machine which the engineer has at his com- 

 mand. Eight years ago, when Dr. Hopkinson determined 

 by experiment the efficiency of an Edison-Hopkinson 

 machine, he was able to show that when mechanical energy 

 was imparted to the shaft of the armature at the rate of 57*6 

 H.P., electrical energy was delivered at the terminals in a 

 useful form at the rate of 537 H. P., or an efficiency of over 

 93 per cent. In the waste power is included the loss due 

 to the passage of the current through the resistance of the 

 armature, the energy required for the excitation of the mag- 

 net, and the mechanical loss by friction of the bearings, and 

 when we consider that all these losses must necessarily be 

 present, however much we may succeed in minimising their 

 amount, it will be seen how little room there is for improve- 

 ment on the score of efficiency. Hence, in point of fact, 



1 " Notes on the Design of Multi-polar Machines" (Esson), Jour?ia/ Inst. 

 El. Eng., vol. xx., part xciii. 



