﻿278 The Clark Cell when Producing a Current. 



fall of 7 units in the hour, and from the table § 6 we have 

 the polarization on immediately making contact equal to 29 



units ( ki)(\(\ )> or 58 of our present units. 



The polarization, or — (E— e), for a current-density of 



ir ^ c — K — 2 ^ or ce ^ -^ ma 7 a t an y time be expressed 



approximately by 



-(58 + 70, 



where t is the time in hours, and the unit of E.M.F. is 



Clark -_ , . .. ,, . . 



n nnn . Expressed in volts this is 



-(•00834 + -001 0- 



It is deduced from experiments on cell L, and only indicates 

 the general order of magnitude of the effect with the other 

 cells. 



§ 8. Conclusion. 



Throughout the calculations it has been assumed that the 

 total electromotive force round the circuit is equal to that of 

 the cell at rest ; and therefore that the electromotive force 

 overcoming polarization can be obtained by subtracting from 

 the electromotive force of the cells at rest the observed cur- 

 rent. With the exception of small Peltier effects, I know of 

 nothing against this assumption. 



The results of the experiments are : — 



(a) The electromotive force of polarization varies directly 



with the current-density in a particular cell. 

 (/>) The electromotive force of polarization slowly in- 

 creases when the current is maintained. 

 From the magnitudes of the quantities found in these 

 experiments, it follows that small currents of approximately 

 known value can be obtained by the use of large Clark cells 

 of small internal resistance, which may be neglected in com- 

 parison with the large external resistance. The experiments 

 show that the large cell L when producing *01 ampere was 

 not at all disturbed. This corresponds to a current-density 



— \ — r- . Now in a Board-of-Trade pattern Clark cell 



Do (centim.) 2 r 



there is generally an exposed area of zinc equal to 2 (centim.) 2 , 

 and as the current taken is never greater than innnn 



a 10,000 ohm coil being used in series with it, it follows 

 that the current-density with this value of current is very 



