EFFICIENCY OF ELECTRIC GENERATORS. 95 



Table 1 gives the results of 12 experiments conducted under 

 conditions of maximum of speed. 



The machine not being able to practically produce an 

 intensity superior to 25 amperes, has a maximum capacity of 

 3325 watts, or 332*5 kilogrammetres, with an efficiency of 

 0*75. The external resistance E for this maximum capacity is 

 equal to its internal resistance multiplied by 4 '5. Had we 

 gradually reduced the external resistance until R = r, we 

 should have raised the temperature of the bobbin to such a 

 degree that the insulating substances would have been carbonised 

 and the machine destroyed in a few seconds. 



The examination of the electrical and mechanical efficiencies 

 will show that the greatest electrical efficiency corresponds to 

 the smallest mechanical efficiency. This fact is explained by 

 the constancy of the work absorbed by the frictions and the 

 driving band, which work has the greater influence upon the 

 mechanical efficiency in proportion of the external capacity 

 being weaker. 



The maximum mechanical efficiency of 0'778 corresponds 

 to 18 amperes and 129 volts, that is to say, to a capacity of 

 2342 watts, and the internal resistance is the sixth of the ex- 

 ternal resistance. It is to be observed that for a maximum 

 capacity of 3325 watts, this mechanical efficiency is 764, or 

 very near the preceding one ; that consequently the best use of 

 this machine is that in which R = 4'46r;C=25;E = 133. 



This being established, we have reduced the speed so as to 

 be able to close the circuit on an external resistance equal to 

 that of the machine. 



The new speed was 550 revolutions per minute ; the work 

 absorbed by mechanical frictions was reduced to 10 kilogram- 

 metres per second. 



We made twelve observations, causing the external resistance 

 to vary from 3 875 ohms to 079 ohms. The results obtained 

 are shown in table No. 2. The maximum capacity happened to 

 exactly correspond with the equality of internal and external 

 resistances ; the total capacity, which in the first series of experi- 

 ments was 3325 watts, was reduced to 648 watts, or five times 

 less than with the practical normal speed of 1750 revolutions. 



