﻿272 Mr. S. Skinner on the Clark Cell 



In all three cells the zinc plate, a piece of common zinc 

 sheet, is placed horizontally, and they have the same E.M.F. 

 within 2 parts in 5000. 



§ 3. The Efect of Uniting the Poles of a Cell. 



Let a cell of electromotive force E and internal resistance 

 R have its poles joined by a wire of resistance r ; then, 

 providing R and r are constant, and there is no polarization, 



the potential- difference between the poles will be t> , • -^ 



however, there is polarization, then this potential-difference 



will be ^ — , where e is the value of the electromotive force 

 K-f r 



required to produce the observed current. 



The value of — (E— e) is the electromotive force of polari- 

 zation. 



In the large Clark cells described above E and e are nearly 

 equal when the current is not larger than '01 ampere ; this 

 small difference is the subject of the following measurements. 

 To obtain it, it is clearly necessary to find the value of the 



potential-difference ^ , and of the resistances R and r. 



The measurements for R are contained in Sect. 4 ; r 

 consisted of thick wire coils or of a special wire resistance 

 immersed in a large tank of paraffin oil ; and the measure- 

 ments of the potential-differences are contained in Sect. 6. 



We shall now give an illustration, taken from an actual 

 experiment, of what happens when the poles of a cell are 

 united. On July 25th the cell L had an electromotive force, 

 with its poles open, represented by 5009 on the compensator. 

 The poles were then joined by a resistance of 1000 legal ohms, 

 and their potential-difference, taken as quickly as possible, 

 was then rather more than 4983 and rather less than 4984. 

 Then a resistance of 500 legal ohms was substituted for the 

 1000 ; the potential-difference fell to 4959. Next, with 200 

 legal ohms the potential-difference was 4885. Lastly, the 

 poles were opened, and the electromotive force of the cell was 

 found to be 5009. 



It is seen from these numbers that the cell recovered its 

 electromotive force entirely after the various operations, and 

 this is a typical instance of the behaviour of the cells. It 

 should be mentioned here that these tests, as well as tests on 

 two other cells, were made in the interval between the two 

 sets of resistance observations recorded in the column of the 

 table §4, headed " July 25th, Before— After." 



