CH. X TIME CURVES 215 



first step is 48 amperes, and the acceleration 4'3 f.p.s. per 

 second, and the time 1/2 seconds. On the second step the 

 acceleration is 1*61, the time 5 - 65 seconds, and the total 

 time of starting 7 seconds. So that by putting on larger 

 wheels and increasing v we have reduced the time of 

 starting from 11*5 seconds to 7 seconds. 



It appears, then, that when the resistance to motion 

 consists of frictional torque at the axles, as in motor cars, 

 we can reduce the time of starting by putting on larger 

 wheels and increasing v or M ; but that if the resistance to 

 motion is in the form of draw-bar pull^as with locomotives, 

 we cannot quicken the start by increasing the weight of 

 the motor and the diameter of the driving-wheels. 



The reduction of the frictional and other 

 resistances to motion is thus of great importance. 

 for we can thereby reduce the frictional current and have 

 a greater proportion of the total current available for 

 acceleration. 



Let T be the total tractive effort produced by a given 

 current, T f the frictional and other resistances of a train 



rp _ f 



weighing W l tons. The acceleration will be K * f.p.s. 



1/ Q V V 



per second. 



If the frictional resistances are reduced to -T f , we can 



f- 



get the same acceleration as before with a train weighing 



TF 2 tons, where W 2 is given by the equation 



.................. (97). 



' 1 

 This equation tells us by how much we can increase 



