CH. X TIME CtTEVES 211 



1,800 amperes. The tractive effort required for the train 

 is 9 pounds per ton of load. 



The given weight includes that of the locomotive, 

 and the tractive effort required for the latter is to be 

 reckoned in the same way as for the train ; each motor 

 has then to draw 195 tons with a tension of 156 g 2 

 volts. 



The draw-bar pull per motor for the grade is 3,490 

 pounds, and for friction 1,755 pounds, making altogether 

 5,245 pounds. If we allow 95 per cent, as the mechanical 

 efficiency of the motors, the value of T to be inserted in 

 Equation 94 is 5,500 pounds ; using the given values of 



M 



R, 8, E. and v, we find that - - = 2 -33. We have thus a 



cL 



wide range of possible values of M and c?, any of which 

 would satisfy the required conditions, but whatever values 

 are taken we shall find the time of starting up to be the 

 same. 



We might, for instance, take M = 150 and d= 64 inches, 

 the friction current would then be 795 amperes, and the 

 accelerating current 1,005 amperes. From Equation 93 

 we find the acceleration to be 0'49 f.p.s. per second. The 

 speed when the rheostat is all out is 13'3 f.p.s., and the 

 time occupied on this step is, therefore, 27 seconds. The 

 time factor is 4*8 seconds from Equation 87. If we 

 consider the start to have been effected when the speed is 

 0'98 of the final speed, the time must be increased by 2-r, 

 say by 1 seconds, so that the train will have been started 

 in 37 seconds from rest. Nothing is gained by increas- 

 ing the induction factor or the diameter of the driving- 

 wheels. The values actually used are: -/If =144 at 800 

 amperes, and d=6'2 inches. 



p 2 



