370 Prof. W. E. Ayrton and Mr. W. R. Cooper. 



throughout for balancing the cells under test, and it was compared 

 at frequent intervals with the other two in the ice bath. The differ- 

 ences between these were remarkably constant, so that it was probable 

 that the standard did not change to any great extent. 



Experiments were conducted upon several cells at a time. These 

 were placed in a water bath surrounded by an outer vessel, also con- 

 taining water. Heat was applied, and time readings were taken of 

 the temperature of the inner bath and of the differences of E.M.F. 

 between each cell and the standard. The initial temperature was 

 always that of the room, and, as there was an interval of at least one 

 night between successive experiments, the cells were presumably in a 

 fairly steady state before any heating began. The results were 

 plotted in the form of curves, some of which are reproduced, having 

 time as abscissae, and temperature, or variations of E.M.F., as 

 ordinates. In fig. 1 is shown the effect on four cells of a rise of 

 13*4° C. in about four hours, after which the temperature was main- 

 tained approximately constant for nearly one hour and a half, and 

 then allowed to slowly fall. The upper curve shows exactly how the 

 temperature was changing at anytime; and the lower carves the 

 corresponding effect on the E.M.F. of each of the cells. It will be 

 seen that each of the latter is similar to the temperature curve, the 

 irregularities of which are reproduced more or less closely according 

 to the sensitiveness of the cell. All the lower curves are, however, 

 displaced somewhat to the right, compared with the temperature ; 

 that is, a change of curvature, or a maximum in E.M.F., is generally 

 reproduced somewhat later than it occurs in the temperature curve. 

 There is, in fact, as would be expected for such a rate of variation 

 of temperature, a lag which is not the same in value for all the cells. 

 It will be observed that every cell curve drops almost as soon as the 

 temperature begins to fall. 



These preliminary experiments all tended to show that cells re- 

 sponded very readily to variations of temperature. In fact, a glance 

 at the curves would lead one to think that lag was negligible, and 

 that cells reached a practically steady state after a few minutes of 

 steady temperature, any further small changes of the latter being 

 reproduced without delay. But these results can only be regarded as 

 approximate, and leave out of the question any possibility of what 

 may be termed a semi-permanent lag, by which we mean any slow 

 settling down after a large variation of temperature, before a cell 

 attains a really steady condition. It, therefore, appeared desirable 

 to carry out further experiments with increased precautions to ensure 

 accuracy. 



For this purpose three cells were again used as a standard, and 

 were placed in boiling tubes fixed in a water bath, but instead of 

 putting ice into the latter it was surrounded by an outer vessel into 



