ELEMENTARY THEORY OF ELECT ROMAGNETISM. 71 



7. The core of an induction coil carries 100,000 lines of mag- 

 netic 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 

 required in the secondary coil in order that an average electro- 

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

 the 0.003 second? Ans. 83,333 turns. 



8. The ring armature of a direct-current bipolar dynamo has 

 260 turns of wire upon it, the armature is driven at a speed of 

 i ,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. 1144 volts. 



9. A circular metal disk DD is 10 centimeters in radius and is 

 mounted on an axle in a long hollow coil 55 having 5 turns of 

 wire per centimeter of length as shown in Fig. py. Two brushes 

 maintain contact with center and edge of the disk, and the 

 galvanometer gives no deflection when the disk runs at a speed 

 of 75.2 revolutions per second. What is the resistance of the 

 coil R? Ans. 1,485,000 abohms. 



n 







pulley 



{ TV V y V ^^ 







Fig. p9. 



Note. Find intensity of the magnetic field in the very long coil 55 for an 

 assumed current / in the coil. Then find the magnetic flux through the disk DD. 

 This amount of flux is cut per revolution by every radius of the disk, and multi- 

 plying by the revolutions per second gives the electromotive force between the 

 brushes; and this electromotive force balances the electromotive force across the 

 resistance R. 



The arrangement shown in Fig. pg was devised by L. Lorenz in 1873 for the 

 "absolute" measurement of resistance; that is, for the measurement of resistance 

 directly in mechanical terms. 



10. A transformer takes alternating current from supply mains 

 at 1,100 volts and delivers current to service mains at no volts. 



