SCIENCE AND PRACTICE. 315 



the other through the shunt resistance, which is equal to 

 their sum. The consequence is that the intensity of the 

 current in the galvanometer branch is decreased, while the 

 intensity in the whole circuit is increased to 





2 



|3 being the new deflection due to this altered state of things. 

 The shunt is taken away, the needle returns again to a and 

 the current has its original intensity, F (a). The resistance 

 r of the rheostat is then increased by r, until the needle 

 descends from ct to j3, with the corresponding intensity. 



Dividing (I by (II, the value of x is obtained. 



55. Determination of the Electro-motive Forces of Galvanic 

 Elements. In calculating the insulation resistances, and other 

 electrical conditions of submarine cables from the deflections 

 of a needle, it is indispensably necessary to know the electro- 

 motive force of the battery used in the measurement. The 

 methods of comparing the electro-motive force of a battery 

 with that of some constant element, taken as unit, are very 

 various. Sometimes the electro-motive forces of elements 

 are referred to the amount of work which, with an unit of 

 resistance in the circuit, they are capable of performing in 

 an unit of time ; as, for instance, the measurement by the 

 voltameter. 



The intensity I of the current in any closed circuit is 

 directly proportional to the volume of water which it decom- 

 poses in a given time, and when the resistance R is constant, 

 the volume decomposed is also proportional to the electro- 

 motive force, for when the unit of resistance is determined 

 upon, or R = 1 is set in the expression of Ohm's fundamental 



E 

 equation, I = , we get the equation I = E, that is to say, 



