432 PROBLEMS AND EXERCISES. 



ometer-coil. Show by calculation (a) that the internal resist- 

 ance of the batteiy is equal to that of 40 miles of the line-wire ; 

 (b] that to produce an equal deflection at a station 360 miles 

 distant the number of turns of wire in the galvanometer - coil 

 must be 40. 



6. Suppose an Atlantic cable to snap off short during the 

 process of laying. How can the distance of the broken end 

 from the shore end be ascertained ? 



7. Suppose the copper core of a submarine cable to part at 

 some point in the middle without any damage being done to 

 the outer sheath of guttapercha. How could the position of 

 the fault be ascertained by tests made at the shore end ? 



8. Explain the construction and action of an electric bell. 



9. Describe and explain how electric currents are applied in 

 the instruments by which very short intervals of time are 

 measured. 



10. Explain the use of Graham Bell's telephone (i) to 

 transmit vibrations ; (2) to reproduce vibrations. 



11. Describe a form of telephone in which the vibrations of 

 sound are transmitted by means of the changes they produce in 

 the resistance of a circuit in which there is a constant electro- 

 motive-force. 



12. Two coils, A and B, of fine insulated wire, made exactly 

 alike, and of the same number of windings in each, are placed 

 upon a common axis, but at a distance of 10 inches apart. They 

 are placed in circuit with one another and with the secondary wire 

 of a small induction-coil of RuhmkorfFs pattern, the connections 

 being so arranged that the currents run round the two coils in 

 opposite directions. A third coil of fine wire, C, has its two 

 ends connected with a Bell's telephone, to which the experi- 

 menter listens while he places this third coil between the other 

 two. He finds that when C is exactly midway between A and 

 B no sound is audible in the, telephone, though sounds are 

 heard if C is nearer to either A or B. r Explain the cause of this. 

 He also finds that if a bit of iron wire is placed in A silence is 

 not obtained in the telephone until C is moved to a position 

 nearer to B than the middle. Why is this ? Lastly, he finds 

 that if a disc of brass, copper, or lead, is interposed between A . 

 and C, the position of silence for C is now nearer to A than the 

 middle. How is this explained ? 



