CH. IX CONTEOL 199 



The specification, however, called for a definite maxi- 

 mum speed namely, 30 miles per hour, and we have 

 already seen that when the final speed is given, the 

 greater the internal resistance, the smaller will be the 

 induction factor, and consequently the smaller will be the 

 torque for any given current. The induction factor of 

 these motors with 100 amperes in the series-wound 

 magnets is 172, giving a tractive effort of 1,460 pounds 

 per motor on 33-inch wheels. If the resistance had been, 

 say, 0'387 ohm per motor, the induction factor to run 

 at the given speed might have been 184 at 100 amperes, 

 giving a tractive effort of 1,560 pounds. The high resist- 

 ance thus involves a loss of over 6 per cent, in the 

 starting torque. Some motors recently designed for this 

 line have a resistance of 0'83 ohm (vide Paper by 

 Mr. S. B. Cottrell published in the ' Electrician ' of 

 October 9, 1896). 



The motor cars of the City and South London Railway 

 are each equipped with two series-wound motors controlled 

 on the series principle. Diagrams of current and accelera- 

 tion, the results of experiment, have been given in Fig. 41. 

 The weight of the train is 35 tons ; the tension of the line 

 is 400 volts. The motors are gearless, with an internal 

 resistance of 0'387 ohm each. As these motors have to 

 make up their speed in series, the induction factor will be 

 less than half that of the Liverpool motors. The value of 

 this factor for 100 amperes is 61 (see Fig. 39) and the 

 tractive effort is 640 pounds, the wheels being 27 inches 

 in diameter. 



If we take the friction to be the same as for the 

 Liverpool motors namely, 20 pounds per ton on 33-inch 

 wheels, the tractive effort required to overcome this friction 



