PROBLEMS. 459 



to the axis of the armature, and the field intensity in the air gap 

 between the pole face and the armature core is 6,000 gausses. 

 The wires on the armature are 1 2 centimeters from the axis of the 

 armature, and the speed of the armature is 1,800 revolutions per 

 minute. Find the electromotive force in volts induced in each 



armature wire (30 cm. long) as it sweeps across the pole face. 

 Ans. 4.072 volts. . 



59. The core of an induction coil carries 100,000 lines of flux 

 when current is flowing through the primary coil. When the 

 primary circuit is broken the flux through the core drops to 

 10,000 lines in 0.002 second. How many turns of wire are re- 

 quired in the secondary coil in order that an average electromo- 

 tive force of 5,000 volts may be induced in it during the 0.002 

 second ? Ans. 1 1 , 1 1 o turns. 



CHAPTER II. THE DYNAMO. 



60, A coil of wire wound on one tooth of an alternator arma- 

 ture like that shown in Fig. 16, Chapter II., has 80 turns of wire. 

 When the tooth is squarely under a north pole of the field mag- 

 net the magnetic flux through the coil is 2,500,000 maxwells. 

 The field magnet has ten poles and the armature makes 2 5 revo- 

 lutions per second. Find the average value of the electromotive 

 force induced in the armature coil while the tooth on which it is 

 wound moves from a position squarely under a north pole to a 

 position squarely under a south pole of the field magnet. Ans. 

 1,000 volts. 



