60 THE MAGNETIC CIRCUIT [ART. 25 



the total resistance of the secondary circuit be r ohms. Show that when 

 t!u current in the primary winding is reversed, the discharge through 

 the galvanometer is equal to 2n<P coulombs. 



Prob. 2. A telephone line runs parallel to a direct-current trolley 

 feeder for 20 kilometers. When a current of 100 amperes flows through 

 the feeder a flux of 2 kilo-maxwells threads through the telephone loop, 

 per meter of its length. What is the average voltage induced in the tele- 

 phone line when the current in the trolley feeder drops from 600 to 50 amp. 

 wit hin 0.1 sec.? Ans. 22 volts. 



Prob. 3. Determine the number of armature conductors in series in a 

 550 volt homopolar generator of the axial type, running at a peripheral 

 speed of about 100 meters per sec., when the length of the armature iron 

 is 50 centimeters, and the flux density in the air-gap is between 18 and 19 

 kilolines per sq. cm. Note: For the construction of the machine see the 

 Standard Handbook, index, under " Generators, homopolar." 



Ans. 6. 



Prob. 4. Draw schematically the armature and the field windings of a 

 shunt-wound direct-current generator, select a direction of rotation, and 

 show how to connect the field leads to the brushes so that the machine 

 will excite itself in the proper direction. 



Prob. 6. From a given drawing of a direct-current motor predict its 

 direction of rotation. 



Prob. 6. In an interpole machine the average reactance voltage per 

 commutator segment during the reversal of the current is calculated to be 

 equal to 34 volts. What is the required net axial length of the commu- 

 tating pole to compensate for this voltage if the peripheral speed of the 

 machine is 65 meters per second, and the flux density under the pole is 

 6 kl. sq.cm.? The armature winding has two turns per commutator seg- 

 ment. Ans. 22 cm. 



25. The Induced E.M.F. in a Transformer. The three types 

 of transformers used in practice are shown in Figs. 12, 13, and 14. 

 Considering the iron core as a magnetic link, and a set of primary 

 and secondary coils as an electric link, one may say that the core- 

 type transformer has one magnetic link and two electric links; 

 the shell -type has one electric link and two magnetic links; the 

 combination or cruciform type has one electric and four magnetic 

 links. Still another type, not used in practice, can be obtained 

 from the core-type by adding two or more electric links to the same 

 magnetic link. Each electric link is understood to consist of two 

 windings: the primary and the secondary. 



When the primary winding is connected to a source of constant- 

 potential alternating voltage and the secondary winding is con- 

 nected to a load, alternating currents flow in both windings and an 

 alternating magnetic flux is established in the iron core. If the 



