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



indeed, it is sometimes found not to have completely disappeared 

 by the next morning. In extent it seems to vary considerably, 

 even in the same cell. For example, in the cell F, it is large in 

 fig. 6, small in fig. 8, and zero in fig. 7. It is considerable for both 

 ~W and Z in figs. 6 and 7, but small in fig. 8. 



In these curves no correction has been introduced for any variation 

 during an experiment of the temperature of the cells used as standards, 

 for the error probably did not amount to more than two, or three, 

 ten-thousandths of a volt, and there was some uncertainty as to such 

 variations. The thermometer indicated the temperature of the water 

 in the bath, but the standards being in boiling tubes were sur- 

 rounded by air. It is, therefore, probable that they did not very 

 readily follow any small variations in the temperature of the bath, 

 and it was consequently thought best to introduce no correction. 



In practice we seldom have to deal with such large changes of 

 temperature as 10° C. produced in 2^ hours. In order to observe 

 whether lag existed with slow variations, an experiment was carried 

 out, during which a change of about 2° up and down took place at 

 the rate of 1° in fifty minutes, as shown in fig. 9. Since the corre- 

 sponding change in E.M.F. was rather small, it seemed desirable in 

 this case to introduce a correction for any change that might occur 

 in the temperature of the standard cell. And, since, in consequence 

 of the thin layer of air that separated this cell from the water in the 

 bath, small changes in its temperature might not be accurately 

 recorded by the thermometer in the water, an indirect method was 

 employed for detecting any small changes in the E.M.F. of the 

 standard by comparing it with the E.M.F.s of three cells placed in 

 a water bath and frequently stirred. The standard was compared 

 with these at short intervals during the experiment, and their tem- 

 perature, which rose approximately through 0"54°, was carefully noted. 

 One of these cells, although giving good results, did not appear to be 

 quite as steady as the other two, viz., M and N", which were very 

 consistent. It was accordingly discarded. Curves M and ~N con- 

 necting the E.M.F. differences of these cells with time are plotted 

 in fig. 9. The dotted curve may be considered as their mean. The 

 variation of this from the starting point, or from the base line ab> 

 is due to two causes — (1) change in temperature of M and ~N, (2) 

 change in the temperature of the standard. From the results ob- 

 tained it may be fairly assumed that the cells M and N have not 

 otherwise changed in themselves. If the variation due to change of 

 temperature of M and N can be estimated, that due to change in the 

 standards can then be deduced. But this cannot be done without 

 assuming some knowledge of the quantity we are investigating, viz., 

 taking it for granted that cells are readily affected by small changes 

 of temperature. As, however, this conclusion is supported by much 



