530 SUBMABINE CABLE LAYING AND EBPAIRING. 



Suppose the reading with the standard cell balanced to be 

 315. Then the E.M.F. of the battery is 



, . 10,000 ,, ^ ,^ 



i-4x — =44-5 volts. 



315 



This result is really the dirference of potential at the battery 

 terminals, but, owing to the high resistance of the slides 

 (100,000 ohms), only differs from the actual E.M.F. by an in- 

 appreciable amount. Strictly, the above result should be in- 

 creased by the amount 



h 



100,000 



X44-5 



to give the E.M.F. where h is the battery resistance. 



For example, with a battery of 100 ohms the amount to add 

 would be 



^gX44-6 = 00445volt, 



■which would make the E.M.F. only 44*54 instead of 44*50, or 

 an error of only f\,th of 1 per cent. Even with a battery of 

 as high a resistance as 500 ohms the error is only 0*22 volt, 

 or an error of ^ of 1 per cent. Except in the case of abnormally 

 high-resistance batteries this is quite inappreciable, and may be 

 neglected. 



The tests for E.M.F. described are null methods, allowing of 

 great accuracy and convenience of observation. 



Care must be used in balancing with the standard cell. In 

 the out-of-balance positions the key in this circuit must only 

 be lightly tapped to observe the spot as the slides are being ad- 

 justed. It should be remembered that when out of balance on 

 one side (that is, pointers too far away from zero) the E.M.F. on 

 the slides will exceed that of the standard cell, and the cell 

 will receive a reverse current through it. Also, when out of 

 balance on the other side (pointers too near zero) the slide volt- 

 age will be less than that of the standard cell, and the latter 

 will deliver a current. Neither of these contingencies are good 

 for the cell, but of the two the former is the one most to avoid. 



The best way is to calculate approximately the position of 

 the slides to balance, and then put the pointers a little nearer 

 zero than this, and work up to the right position of balance by 

 tapping the key lightly as required. 



