Variations in the E.M.F. of Clark Cells with Temperature. 377 



about 105°, and raising it again at the rate of one degree every 

 quarter of an hour. Cells F, W, and Z appear to have a smaller lag 

 with a falling than with a rising temperature, whereas there is very 

 little difference in cells C and X. In fig. 7 curves are shown for 

 nearly the same range of temperature, but this was maintained for a 

 longer time near its lowest point. Cells W and Z give curves 

 similar to those in fig. 6. J has a smaller lag for rising temperature, 

 and F appears neutral, though enclosing a large area. Fig. 8 shows 

 curves obtained when the temperature was first raised and then 

 lowered through about 8'5° at the rate of one degree per quarter 

 of an hour. Unfortunately we were accidentally prevented from 

 taking observations daring one part of the time, but there is little 

 doubt that the curves follow pretty nearly the course marked. It is 

 noticeable that cells F and perhaps W appear to have a slightly 

 smaller lag with a rising than with a falling temperature, whereas 

 the reverse is the case in figures 6 and 7. Cell L has a similar 

 curve, and Z is now neutral, though not so formerly. Inequality of 

 lag for rising and falling temperature thus appears to vary from cell 

 to cell, and even to vary largely in any one cell, as shown by the 

 curves for cell B and others. The following table gives these results 

 in a manner more easy for comparison : — 



Conditions of experiment. 



Lag smaller for 

 rising 

 temperature. 



Neutral. 



Lag smaller for 

 falling 

 temperature. 



Fig. 5. Fall and rise through 9'5° 

 Fig. 6. „ „ 10-5° 

 Fig. 7. „ „ 10-0° 

 Fig. 8. Rise and fall through 8*5° 



Fig. 9. „ „ 2-0° 



B, L (slight), M 

 j 



B, F (slight), 

 L (slight), W (?) 

 F,L 



Gr, N 



C, X 

 F 

 Z 



w, z 



F, W, Z 



w, z 



It will also be observed that the curves between temperature and 

 E.M.F. are seldom closed, even though readings were continued for 

 some time after the former assumed a steady value, as may be seen 

 by referring to the curves relating to temperature alone. At the end 

 of any experiment the cells became practically steady after about 

 twenty minutes, and would then readily follow any subsequent varia- 

 tions, as, for example, in fig. 6. The slow rate at which cells assume 

 a really steady state is shown by the curves which have been plotted 

 between E.M.F. differences and time in the lower right-hand corner of 

 figs. 5, 6, 7, 8, and which refer to the conclusion of each experiment. 

 One hour and a half is generally quite insufficient to remove the 

 semi-permanent lag for this rate of variation, as shown 1 in fig. 8; 



