150 SCIENCE PROGRESS. 



although the efficiency above recorded has often been re- 

 peated and is now guaranteed under ordinary commercial 

 conditions, it has seldom been surpassed, at least in machines 

 of similar comparatively small size. The method devised 

 by Dr. Hopkinson for testing the efficiency of dynamos is 

 worthy of passing mention. Two similar dynamos are 

 taken and are connected together either by a coupling or by 

 belting ; one of these, A, is arranged to work as a generator, 

 and the current which it gives is supplied to the other, B, 

 which consequently works as a motor. The mechanical 

 energy delivered by the latter supplies the greater part of 

 the driving power required by A ; the horse-power of B 

 as a motor must, however, necessarily be less than the 

 horse-power required to drive A as a generator. This de- 

 ficiency has, therefore, to be made up from some external 

 source of power, and in the original experiments of Dr. 

 Hopkinson it was supplied through a transmission dynamo- 

 meter, which at the same time measured it. Thus the 

 whole system is self-supporting, save for the necessary 

 losses in the two machines under test, and from the power 

 supplied to make up these losses, the efficiency of the two 

 machines is easily deducible. The beauty of the method 

 lies not only in the fact that, e.g., two dynamos of fifty horse- 

 power each may be tested under full load by means of a ten 

 horse-power engine, but also in the directness with which 

 the waste power is measured. A subsequent improvement 

 of the same method consists in the use of a small subsidiary 

 dynamo to supply the waste power electrically to the circuit : 

 all the measurements can then be made electrically by volt- 

 meter and ammeter, and a very high degree of accuracy is 

 thus obtained. 1 When the dynamos of 240 kilowatts out- 

 put at the Manchester central electric light station were 

 tested by this method, efficiencies of 937 and 95*2 per cent, 

 were found to have been obtained. 2 Of late years, owing 

 to the demand for dynamos coupled directly to the crank- 

 shafts of their driving engines for central-station work, much 



1 "The Determination of the Efficiency of Dynamos" (Kapp), Electrical 

 Engineer, 22nd and 29th Jan., 1892. 



2 Electrician, 6th Oct., 1893. 



