SCIENCE AND PRACTICE. 313 



Committee, which, probably suggested the idea of the ad- 

 justing wire in the British Association balance. 



53. Determination of the Constants of Galvanic Elements. 

 We have already seen that, according to Ohm's law, the 

 intensity of the current in any galvanic circuit is a function of 

 the electro-motive force, and resistance in that circuit ; or that 



I- E 

 K" 



R, being the sum of all the resistances, and E the sum of all 

 i;he electromotive forces in the circuit ; and have considered 

 the value of B, as the sum of resistances interior and exterior 

 to the battery, by which the same value of I is expressed by 



r being the resistance due to the passage of the current from 

 the platea of the elements to the fluids, and vice versa ; and r l 

 that which is exterior to the element interposed resist- 

 ance. 



Resistances we can compare directly, or calculate from 

 given dimensions and conducting powers of materials ; in- 

 tensities we can also compare directly with each other, or 

 with some given amount of work done in the decomposition 

 of water or salt solutions, or in the deflection of a magnetic 

 needle, or in heat developed ; and electro-motive forces we 

 can compare with each other by their known relations to 

 these two combined. 



54. Determination of the Resistances of Galvanic Elements. 

 There are different ways in which this may be done. The 

 readiest is with the aid of a tangent galvanometer. The 

 element whose resistance is to be measured, is put alone in 

 the circuit of the galvanometer, the deflection of whose 

 needle is observed ; a resistance is then inserted in the cir- 

 cuit, which lessens the intensity of the current and diminishes 

 the deflection of the needle to $' degrees. The two inten- 

 sities are 



C. tan. = 



