NOTES A AD APPENDICES. 307 



contradiction to that which has often been made, that the 

 duty is maximum when the external resistance is equal to 

 the internal resistance. There is in this an error in the 

 words which must be rectified. 



It is not the duty that is maximum in this last case, for it 

 is only 50 per cent., but the utilizable work in the external 

 circuit. The largest quantity, therefore, of utilizable elec- 

 tricity will be obtained from a -given machine by making the 

 external resistance equal to the internal resistance, but the 

 highest electrical duty will be obtained by making the external 

 resistance equal to 5 or 6 times the internal resistance. 

 Under these conditions the machine will supply very little 

 electricity, but the greatest part of it will be utilized on the 

 external circuit. In practice, it is preferred to lose on the 

 duty, and to cause the machine to produce the most that it 

 can furnish at its normal velocity, by placing it under the 

 conditions of maximum utilizable work, a maximum which 

 is attained when the external circuit is equal to the internal 

 resistance, certain corrections being made, which, according 

 to the experiments of Jamin, Roger, and Le Roux, must be 

 introduced into the value of the internal resistance. 



In one experiment, the machine turning with a velocity of 

 1,000 revolutions per minute, with an external resistance of 

 27 ohms, or i '8 times the internal resistance, a current was 

 produced of 25-5 webers intensity, with an electro-motive 

 force of 107 volts. The total work transformed into elec- 

 tricity was 273 kilogramme tres, or 3-64 horse-power; the 

 utilizable work was 179 kilogrammetres, or 2*38 horse- 

 power. 



In this case the duty reached 65 per cent. By taking into 

 account friction, passive resistances, &c., the value of which 

 might reach i horse-power, the utilizable work is only about 

 50 per cent, of the work really used up by the machine. 



Experiments made under various conditions of resistance 

 on different machines manufactured at Silvertown gave 

 similar results. 



20 2 



