532 



SUBMARINE CABLE LAYING AND REPAIRING. 



divisions (witti zero at one end) any voltage up to 6 can be 

 measured, say, one to four cells. If larger batteries are to be 

 measured, say, anything up to 20 cells, a range of 20 divisions 

 per volt would be suitable. This is obtained by adjusting the 

 shunt so that the standard cell gives a deflection of 1"435 x 20 

 =say, 28*7 divisions. The full 600 divisions on the scale would 

 then represent 30 volts. Or if, for example, a 15-cell battery 

 gave a deflection ofj 480 divisions, the E.M.F. of this battery 

 480 



would be 



20 



= 24 volts = 1 '6 volts per cell. 



When the deflection of the unknown battery is taken it ia 



' Standard Cell 

 or Battery to be tested 



Fig. 300. — E.M.F. by Direct Deflection. 



connected to the key in place of the standard cell. For a 

 given range on the scale the galvanometer shunt must, of course, 

 be the same for both battery and standard cell deflections. 



Measurement of Galvanometer Resistance. — The resistance 

 of a galvanometer, the temperature at which it is correct and 

 the coefiicient are usually marked inside its containing box by 

 the makers or accompany the instrument in some way, but 

 it happens sometimes from some cause or other that the record 

 is not available when wanted. It then becomes necessary 

 to make this measurement either for the above reason or 

 because for an accurate test it is considered advisable to check 

 the resistance at the temperature then prevailing. 



