ill. IV i||r\T-WnfNI> MOP Cil 



inch-|x>und, tin- sjfc-ed will be 1.1"< r.p.ni. If the 

 roriitnnrr is increased by 0-8 ohm. the speed will fall to 

 888 revolutions per minut.-. 



If the resistance be reduced suddenly. tin- ctirn-n! that 

 will flow through the motor will depend ujxui tin* speed 

 at which it was running before tin- resistance was altered. 



Example 22. If the resistance of 0-8 ohm in 

 Exam pi L'l i- taken ..ut Middetdy while the mot<r is 

 nuiiiiiiir at JSs r.p.m.. the induced tension being then 

 21 volt*. tin- current at tin* moment of cutting out the 

 resistance will U> ISO amperes. To find the speed at 

 which the motor IIIUM run before it would In- safe to take 

 the extra resistance entirely nut, so that the current shall 

 not exceed, say, 150 amperes, we must use the K< (tuition 

 being here 150 amperes, ami /.' \-'mx "_' oh in. 

 We find the repiired speed to be 1.0O8 r.p.m. ; the resist- 

 ance must therefore be gradually reduced as the motor 

 speeds up until it runs at this speed, when the rheostat 

 may be cut out altogether. In this way we can deter- 

 resistance of each step of a rheostat that can 

 be used with a given motor, so that the current shall not 

 exceed s certain fixed amount . 



A rheostat is sometimes placed in series with a nmt-.r 

 to obtain a variation in the Hpeed. When thin is done, 

 care must be taken that it is t it pl.-< ed UMween the main 

 terminal)* and the magnet- if the m<>t<>r i- -hunt w<.und,aa 

 in that case we should of course reduce the value <>f the 

 induction fact, r 



If /,' i- the whole resistance in the circuit of a motor 



when running at n revolutions on a line having a tension 



volts, the current being constant and equal to c 



amperes, the whole resistance tt, required in the circuit in 



