360 Messrs. Threlfall and Pollock on the ClarJc Cell 



the large cell has the value of 1*43588 Rayleigh volt at 

 15° C, and the hospital cell the corresponding value of 

 1*434608 volt under the same conditions. 



The second point referring to the effect of variable zinc and 

 mercury surface on the observed values of A with varying 

 currents was practically the first one to be investigated. 

 Without going into the large amount of numerical information 

 collected, we may state the following results : — 



When the change of A during five minutes after short- 

 circuiting is less than or equal to about j-q per cent, of its 

 value immediately after short-circuiting, the currents stand in 

 about the order of the extent of mercury surface exposed. 

 From a study of the small cells amongst themselves, it was 

 found impossible to give any satisfactory explanation (of an 

 experimental character) of the large difference of the maximum 

 possible currents (i, e. those leading to a change of P.D. less 

 than t ] q per cent.) that were observed. 



The mercury surface was practically the same in all these 

 cells, but the zinc surface varied. The currents did not stand 

 at all in the order of the zinc surfaces. The most important 

 factor appeared to be the thickness of the layer of mercurous 

 sulphate, but it was not the only one. If the layer was less 

 than about a quarter of an inch thick, the cells were bad. 

 The result of it all was that the most important thing besides 

 having plenty of mercurous sulphate is to have large surfaces 

 of mercury and possibly of zinc. For this reason the large 

 cells which it was intended to use in practice received the 

 greatest share of attention, and we shall confine ourselves to 

 describing the phenomena exhibited by them. 



The results of some typical measurements are given in the 

 following tables and curves, which will be found to contain 

 information as to the effect of previous history as well. It 

 may be stated that it was found possible to estimate the value 

 of A within less than half a minute after short-circuiting. 



From our point of view, viz. the applicability of the cell as 

 a source of current of 0*001 ampere, it is immaterial whether 

 there is polarization or not so long as it is not of a creeping 

 character — nor then if the rate of creep is constant. 



The answer to the third question is from the practical point 

 of view absolutely vital and has received most attention. 



Table II. gives the experience we have had. As a result 

 the time finally chosen at which to make an observation is 

 two minutes after short-circuiting. The information con- 

 tained in the Table is also given graphically in Plate XII., 

 where curves are drawn with values of A as ordinates and 

 time as abscissae. 



