140 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



100. The ring armature of a direct-current bipolar dynamo has 260 turns of wire 

 upon it, the armature is driven at a speed of 1,200 revolutions per minute, and the 

 magnetic flux from a pole-face into the armature core is 2,200,000 lines. Calculate 

 the electromotive force of the dynamo in volts. Ans. 114.4 volts. 



101. The armature described in the above problem has upon 

 it 500 feet of pure copper wire 325 mils in diameter. What is 

 the resistance of the armature from brush to brush ? Ans. 

 0.0125 ohm. 



Remark. In a bipolar dynamo the wire on the armature constitutes two paths 

 between the brushes. 



102. A transformer takes alternating current from supply mains at 1,100 volts and 

 delivers current to service mains at 1 10 volts. The primary coil of the transformer 

 has 560 turns of wire. How many turns of wire are there in the secondary coil ? 

 The transformer delivers 250 amperes to the service mains. How much current does 

 it take from the supply mains? A usual allowance in transformer coils is 1,000 cir- 

 cular mils sectional area of wire for each ampere. Find size of wire used in primary 

 coil and in secondary coil of the transformer. Ans. 56 turns, 25 amperes, 25,000 

 cir. mils, 250,000 cir. mils. 



