376 



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



In order to investigate the matter, several cells were subjected to 

 such cycles of temperature. For the results to be of any value it 

 was necessary that the temperature should be varied uniformly, and 

 preferably at the same rate when both rising and falling. A good 

 deal of difficulty was at first experienced, but some excellent tem- 

 perature curves were afterwards obtained by Mr. W. Gr. Rhodes, to 

 whom we are much indebted for assistance rendered throughout this 

 tedious part of the work. If the temperature slope became seriously 

 irregular it was readily detected upon plotting the curve of tempera- 

 ture and time, and the E.M.F. readings corresponding with any such 

 irregular periods were then neglected in plotting the curves between 

 the temperature and E.M.F. 



The curves for E.M.F. differences that are reproduced should be 

 considered along with the corresponding temperature curves, which 

 in all cases are drawn underneath the former. Rather a large 

 selection is given so as to avoid the formation of any erroneous 

 conclusion. It will be noticed that the parts of an E.M.F dif- 

 ference curve relating to rising and falling temperature are 

 seldom parallel. It might be urged that this is due to residual 

 changes of temperature making themselves felt on the return 

 curve. But all our experiments tend to show that lag (ex- 

 cluding semi-permanent lag) for this rate of variation of tempera- 

 ture does not, as a rule, exceed a quarter of an hour. The curves 

 at either end would be affected in this manner for a length corre- 

 sponding with about one degree (the rate of variation being about 

 one degree every quarter of an hour). Beyond that errors might 

 arise amounting to one, or two, ten-thousandths of a volt, due to semi- 

 permanent lag, but they would be insufficient to change the general 

 character of the curves. 



In order to avoid confusion, the cell curves have been separated 

 when necessary by displacing the origin of E.M.F. differences, so that 

 different sets of figures along this axis correspond with different cells, 

 as shown by the distinguishing letters above them. It must be 

 remembered that all the cells included in any one diagram were sub- 

 jected to precisely the same treatment and tested under precisely the 

 same conditions. 



These curves indicate that the lag has generally a different value 

 for rising and falling temperature. The curves in fig. 5 were obtained 

 by lowering the temperature about 9*5°, and then raising it through 

 the same range, but somewhat more rapidly. The lag appears to be 

 smaller for a rising temperature in the case of cells B, M, and L, the 

 difference being but slight in the last. Cells G and N are practically 

 neutral in this respect, although it is to be observed that all the cells 

 were in the same bath. 



The curves in fig. 6 were obtained by lowering the temperature 



