THE SMITHSONIAN INSTITUTION. 347 



polarization is greater in the coniLination of zinc and copper than in 

 tliat of zinc and iron. 



This galvanic pohirization we will consider hereafter more at 

 length; it is only mentioned here so far as is necessary to show the 

 course of Poggcndorff's investigation. 



If tlic values found by Ohm's method for the electro-motive force 

 do not accord with the tension series, the cause, as above remarked, is 

 purely in the modification which the original electro-motive force 

 undergoes by polarization. Poggcndorff endeavored to determine tlie 

 value of their original electro-molive force hefore it loas modified by 

 polarization. We will pass by the earlier efforts by which this ob- 

 ject was but imperfectly attained, and turn to the consideration of a 

 method which Poggendorff has published in volume LIV of his 

 Annals, page 161. 



This method differs essentially from all others, in that not the cur- 

 rent of a battery, but only tlie tendency towards a current, is measured. 

 To avoid polarization, Poggendorff endeavored to prevent the current 

 from coming into action, and to compensate it beforehand by another 

 whose electro-motive force was constant and known. 



The arranging and establishing of this compensating method is 

 described somewhat diffusely by Poggendorff, and on that account is 

 not perfectly clear ; hence I have departed from his mode of presenta- 

 tion, since it has been an object in this report to make it as intelligi- 

 ble as possible. 



In Pig. 13, C represents a constant element — say a Grove's, and I 



another voltaic element, whose 



Fig. 13. 



electro-motive force is less than 

 that of C. The positive poles of 

 both are connected by a conductor, 

 and likewise the negative. In the 

 connexion of two poles of like name 

 a multiplier m is inserted ; the 

 connexion of the other two poles 

 can be broken at a at pleasure, 

 and renewed again. The con- 

 ducting wire a d h closes the con- 

 stant battery C. 



Suppose the element I is pre- 

 cisely equal to C, and the con- 

 nexion at a is made, this combination, then, is in fact nothing else 

 than two elements so connected that they constitute a single element 

 with a double surface ; but, if the electro-motive force of I is weaker 

 than that of C, the actions of the currents are somewhat more compli- 

 cated. 



Denote by — 



I, The resistance of the element C, together with the con- 

 ductors between a and b. 

 V, The resistance of the element I with the condiictors 

 between a and b, the resistance of the multiplier in- 

 cluded. 

 r, The resistance of the conducting wire a db. 



