EFFICIENCY OF ELECTRIC GENERATORS. 



93 



2. MACHINE WITH ELECTRO-MAGNET. 



MAXIMUM CAPACITY OF DYNAMO-ELECTRIC MACHINES. In 

 the selection of a dynamo-electric machine one is guided more 

 by its practical efficiency than by its capacity ; there are, how- 

 ever, some instances where the maximum capacity, that is to 

 say, the quantity of watts which a machine can industrially 

 deliver, is of a superior interest to the efficiency ; for example, 

 in the case of a factory established on powerful and regular 

 watercourses, a machine which costs 10,000 francs and produces 

 30 electric horse-power with an efficiency of only 50 per cent., will 

 be preferable to another machine of the same price, but pro- 

 ducing only 20 electric horse-power with an efficiency of 70 

 per cent. 



For obtaining the maximum capacity of an industrial 

 dynamo-electric machine it is necessary: 1st, to -give the 

 machine the greatest speed at which it can run regularly, con- 

 tinuously, and practically with safety ; 2nd, to close the circuit 

 on an external resistance such that the intensity of the current 

 be at the same time the greatest possible, without any dangerous 

 heating of the inducting and induced wires. 



In the Gramme machines for electroplating, the maximum 

 speed is 1500 revolutions per minute for the type No. 1, 

 1750 revolutions for the type No. 2, and 2000 revolutions 

 for the type No. 3. The most suitable external resistance for 

 these machines corresponds to the internal resistance multi- 

 plied by 7 for the type 1, 6 for the type 2, and 5 for the type 3. 



Some authors, arid notably M. Marchese, engineer at 



