THE SMITHSONIAN INSTITUTION. 349 



This is to l)e considered as a first approximation to the correct ratio 

 hetween r and I. The battery I shouhl now he left open for a time, 

 that it may lose all polarization ; or, what would be better, the negative 

 ])late should he taken out of the liquid, cleaned, and then restored to 

 its place. If a deflection occurs again on closing the circuit, tlie length 

 of the wire a d h must he regulated until the exact proportion is ob- 

 tained. The current which the electro-motive force of the clement I, 

 unmodified by polarization, tends to generate, is compensated, and the 

 value of E' can be computed by equation (4). 



Poggendorff proved his method by ascertaining with it the electro- 

 motive force of constant elements, which could be determined in another 

 manner, and found perfectly accordant results. He obtained^, by Ohm's 

 method — 



The electro-motive force of Grove's element = 25.886 



The electro-motive force of Daniell's element = 15.435 



The Grove's element was then placed at C, and the Daniell's at I, 

 (Fig. 13.) I was 35.03. The equilibrium, above mentioned, took 

 place when r =z 52. G8. For this case we have — 



^ + ^ — 1.668. 

 r 



Hence we get by this method 



£' = 2^^ = 15.51, 

 1.668 ' 



which accords very well with the value of E', determined by Ohm's 

 method. 



Poggendorff now used this method for determining the original 

 dectro-motive force in constant batteries. That of Grove's battery, 

 adopted as the standard of comparison, was found by Ohm's method 

 to he equal to 22.88 , and he found for the original force of an incon- 

 stant battery, made of 



Zinc and copper 13. YO 



Zinc and iron ^AO 



Iron and copper 6.00 



These results prove that the original electro-motive force of these 

 combinations very nearly satisfy the law of the tension series, since 

 that of copper and iron, and that of iron and zinc, is nearly equal to 

 the electro-motive force of copper and zinc ; thus, 7.-1 -|- 6 =13,4, 

 nearly equal to 13.79. 



If the current of the zinc and iron battery is stronger than that of 

 the zinc and cojjper, and if, according to Ohm's method, the electro- 

 motive force of the former combination is found greater than that of 

 the latter, it is solely because the current of the zinc and copper com- 

 bination generates a stronger polarization, acting against the original 

 electro-motive force, than the current of the zinc and iron battery. 



§ 14. Comparison of different voltaic combinations. — In the last 

 paragraph we have seen how the constants of a voltaic combination 

 can be determined and expressed in comparable values. None of the 

 statements of the effects of batteries, as they are ordinarily presented 



