266 APPLIED SCIENCE 



ductor, generates heat, representing a further loss, analogous to 

 that resulting from friction in mechanical gearing. This loss, 

 depending on the distance of transmission, the size of the con- 

 ductor, and the electromotive force employed, is easily com- 

 puted. 3rd, we re-convert the electricity into mechanical power 

 by means of an inverted dynamo, which we term an ' electric 

 motor.' With motors in which large weights of iron and copper 

 are employed, the loss in re-conversion need not exceed 20 per 

 cent., but with light motors, weighing from 70 Ibs. to 100 Ibs. per 

 horse-power, such as we must employ in the locomotives, I could 

 not undertake with certainty at this moment to effect the re- 

 conversion without a waste of one-half. The effect of all these 

 sources of loss is, that at the stationary engine I must exert 

 about 3 horse-power for every single horse-power which is em- 

 ployed usefully on the line. I look forward confidently to the 

 time when 2 horse-power at the engine will be sufficient to give 

 1 horse-power to the motor. To put these conclusions in a more 

 scientific form, I may assume the efficiency of my dynamo as 

 80 per cent., that of my small motor as 50 per cent. 1 The waste 



ri2T? 



by heat expressed as horse-power is equal to where C is 



the current in amperes, and R the resistance in ohms. The 



~p P 



horse-power represented by the current is equal to - where E 



74o 



is the electromotive force in volts and C the current in amperes. 

 It follows from the last expression that I may increase the horse- 

 power in three ways, by increasing either E or C, or both. If 

 I increase E, leaving C the same, I do not increase the loss 

 during transmission along the line (except by leakage), no 

 matter what horse-power the given line may transmit. A prac- 

 tical limit is set to the application of this law by the difficulty 

 met with in dealing with electromotive forces above 2,000 volts. 

 Marcel Deprez, taking advantage of this law first pointed out 

 by Sir William Thomson has transmitted seven or eight horse- 

 power over seven or eight miles, through an ordinary telegraph 

 wire, and he obtained a useful duty of 63 per cent., taking into 

 account all three sources of loss which I have enumerated. With 

 small motors I cannot yet promise a result so good as this, and I 



1 The last motor tested, weighing 117 Ibs., made by Mr Reckenzaun, gave 

 l - 72 horse-power with 54 per cent, efficiency. 



