PROBLEMS. 



457 



main A 



main B 



earth 



Fig. 5 IP. 



Supply main 



gives a reading of 2.6 volts and the electromotive force between 



the mains is 1 1 o volts. Find the insulation resistance between 



main B and the earth on the 



assumption that the insulation 



resistance of main A is : (a) 



infinite ; (b) the same as that 



of main B] (c) one-tenth of 



that of main B. Ans. (a) 



660,900 ohms ; (b) 644,900 



ohms ; (c) 500,900 ohms. 

 52. The armature D, Fig. 5 2 



P, of a dynamo is connected to 



supply mains in series with an ammeter A and a suitable rheostat 



R, and the ammeter reads 50 amperes. The terminal leads // 



of a low reading voltmeter V are connected to the two commu- 

 tator segments b and c and 

 the voltmeter reads 4.2 volts. 

 The leads // are then con- 

 nected to brush lead a and 

 segment b and the voltmeter 

 reads I . I volts ; the same 

 reading is obtained when the 

 leads are connected to seg- 

 ment c and brush lead d. 

 Find : (i) the resistance of 

 the armature between com- 

 mutator segments b and c ; 

 (2) the resistance of each 

 brush, including contact re- 

 sistance between brush and 

 commutator ; and (3) the 

 total resistance between a 

 and d. Ans. ( I ) 0.084 



Supply main 



Fig. 52P. 



ohm; (2) 0.022 ohm; (3) 0.128 ohm. 



53, A straight horizontal metal bar 3 meters long falls parallel 



