524 



SUBMAEINE CABLE LAYING AND EEPAIRING. 



To prevent an undue current being passed through the coils 

 before the balance is obtained, it is advisable to make 



rE 



E= 



ohms, 



approximately, and then reduce it as required to balance. E is, 

 of course, given its nearest approximate value in this estimate, 

 but r and e are known. 



For instance, take a battery of 40 Leclanche cells, the 

 E.M.F. of which can be approximately estimated at 65 volts. 

 Putting r=100 and using a standard cell of 1"44 volts, R 

 should at first be made 



100 X 65 



1-44 



= 4,500 ohms. 



and then reduced as required to balance. 



Comparison of EM.F.'s and Potentials by the Slides. — The 

 slides^being useful for the above tests, it may be desirable to 



1-4 Volts 



10 



Standard 



Cell 



Fig. 295. 



briefly explain their use. The action is best explained by re- 

 ference to the potentiometer. This is simply a stretched wire, 

 AB (Fig. 295), of uniform section and any convenient length, 

 say 1 yard, throughout the length of which is an accurately 

 divided scale. A current being passed through the wire, say 

 from A to B, we have a certain difference of potential between 

 the ends, which is distributed uniformly per unit length of the 

 wire, as shown by the dotted lines. For instance, if AB is 



