324 THE ELECTRIC TELEGBAPH. 



is always in the same condition at the moment the observa- 

 tion is made, it follows that we need only alter our phraseology 

 and term the unit of electro-motive force, that of a certain 

 element in those precise conditions of tension, and as the 

 difference between the tensions of the poles of the same kind 

 of element under two given conditions must be always the 

 same, it is easy to deduce from experiment the constant with 

 which the values found must be multiplied, in order to give 

 the first comparison. 



In measuring the electro-motive force of a battery of n 

 similar elements, in series, it will always be found that the 

 observed value is less than n times the observed electro- 

 motive force of each single element. This is explained by 

 the fact that the decompositions and recompositions which 

 take place in the single element are due only to the forma- 

 tion of its current ; whereas, in each of the n elements under 

 consideration, these decompositions and recompositions are 

 due to two causes first, to the formation of its proper cur- 

 rent ; secondly, to the conduction of n 1 times this current 

 that of the remaining elements. This action gives rise to an 

 impoverished solution in the compartment of the negative 

 metal and to a contrary polarisation. 



There are other excellent methods ; but the foregoing are 

 the most important, and the last of them can be recommended 

 as the best. 



63. The following comparisons between the electro-motive 

 forces of pairs of metal plates in a single fluid medium were made 

 by Professor Poggendorff with his method of compensation. 



The alternate relations of three metals were always 

 measured at the same time, all three being placed in the 

 same fluid. It has been explained that the electro-motive 

 force of a pair of metal plates is the difference between the 

 electro-positiveness of the two metals, when immersed 

 in the same fluid. If three different metals are taken 

 in the order of this electro-positiveness, and the differ- 

 ence of the first and second be measured, and then the 

 difference between the second and third, it is evident that 

 the difference between the first and third must be equal to 



