INDUCED jELECTROMOTIVE FORCE. 139 



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Find the electromotive force induced in the wire in volts, the 

 horizontal component of the earth's field being o. 18 gauss. 

 Ans. 0.00135 volt. 



96. The pole-face of a dynamo is 30 centimeters long in the 

 direction parallel to the axis of the armature, and the field inten- 

 sity in the gap space between the pole-face and the armature core 

 is 6,000 gausses. The wires on the armature are 12 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 centimeters in length) as 

 it passes across the pole-face. Ans. 4.07 volts. 



97. A single wire W, Fig. 83, is rotated along the dotted 

 line in Fig. 83 at a speed of 25 revolutions per second. The 

 magnetic flux which emanates from each north pole of the field 

 magnet and which enters each south pole is 2,500,000 lines. 

 (a) Find the average value of the electromotive force induced in 

 the wire during the time that it sweeps from a point midway be- 

 tween two field poles to the next point midway between two 

 poles. (b~) Find the number of cycles per second through which 

 this induced electromotive force passes. Ans. (#) 2.5 volts. 

 () 50 cycles per second. 



98. The alternator specified in problem 97 has a winding as 

 shown in Fig. 84. Find the average value of the electromotive 

 force induced in the winding during the time that the armature 

 is making one fourth of a revolution (that is, during the time that 

 the slots containing the wires travel from a point midway between 

 the pole pieces to another set of points midway between the pole 

 pieces. Ans. 10 volts. 



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

 magnetic flux, when current is flowing through the primary coil. 

 When the primary circuit is broken the flux in the core drops to 

 10,000 lines in 0.003 second. How many turns of wire are re- 

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

 tive force of 25,000 volts may be induced in this coil during the 

 0.003 second? Ans. 83,333 turns. 



