380 



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



evidence, it is probable that curves corrected by a quantity so 

 obtained will be nearer tbe truth than if left uncorrected. 



From previous experiments it appeared that the E.M.F. of M and 

 ~N fell about O'OOIO volt per degree rise of temperature. The ordi- 

 nates of the curve 1\ above the line cd give this rise of temperature 

 from the beginning of the experiment, and are also numerically equal 

 to the changes of E.M.F. Hence, if the ordinates of the dotted curve 

 are diminished by these corresponding ordinates of Ti, another curve 

 ee is obtained, of which the ordinates from ab give the variation of 

 the standard. Since it is wholly below ab, it follows that the E.M.F. 

 of: the standard fell during the experiment, but rose to nearly its old 

 value at tbe conclusion, a result which agrees with the absolute 

 measurements taken. This curve enables the figures obtained from 

 the cells under test to be corrected, the correction, however, being 

 small : they are given in their final form in fig. 9, along with the 

 corresponding temperature curve T 2 T 2 . The changes in E.M.F. were 

 measured to l/50,000th of a volt, and a larger scale is used in this 

 diagram so as to render the differences more apparent. The curves 

 are very similar to those already discussed. Lag is still evident, though 

 of course much diminished. Cells W and Z show practically no lag of 

 a semi- permanent character, but it is strongly marked in L and F, 

 although a steady temperature had been maintained for some time at 

 the end of the experiment. Curve L is noticeable, as showing the 

 uncertain character of lag in a cell, a point which is also exemplified 

 by the curve for B in fig. 8. Other curves of the same kind have 

 been obtained, one of which relates to B, but they have not been 

 reproduced. 



It might be supposed that part of the lag observed was really due 

 to lag in the thermometers themselves. But on subjecting these to 

 cyclical changes of temperature this was found not to be the case. 



In general, it thus appears that Clark cells of this type are very 

 sensitive to changes of temperature, but are subject to a lag, which 

 is variable in value, even when these changes of temperature are 

 small. For that reason it is generally impossible to apply a cor- 

 rection which shall be accurate to 1/1 0,000th of a volt for even very 

 slow variations of temperature. Of course, in work involving a 

 knowledge of the absolute E.M.F. of the cell, this is of no great im- 

 portance, as we do not know the value of the E.M.F. to greater 

 accuracy than 0'1 per cent, in true volts, but it may be of import- 

 ance when dealing with comparisons. 



If we suppose a cell to be placed in a water bath, the temperature 

 of which is 3*0° different from that of the cell previously, we may 

 conclude that after the lapse of half an hour the lag will not amount 

 to more than 0*1 per cent, of the E.M.F., and will generally be less. 

 In such cases the temperature coefficient should be diminished some- 



