126 DR. E. H. CHIFFITHS AND MR. EZER GRIFFITHS ON THE 



The validity of our final conclusions is dependent upon the accuracy with which 

 th.-s,- ,,u.mtitirs NV.TO determined, and in the following sections will be found a 

 description of the methods adopted for their measurement. An error of 1 in 1000 

 in Nos. 1, 2, 4 and 5 supra, and an error of 1 in 2000 in No. 3 would affect our final 

 results by O'l per cent. The thermal capacities of the bodies mentioned in (6), 

 however, were so small, as compared with the capacities of the blocks, that the effect 

 of an error of 1 to 5 per cent, in their valuation would fall below the O'l per cent. 



referred to. 



We have, however, no reason to suspect that errors approaching such limits exist 

 in any of the measurements above enumerated. 



SECTION III. 

 Measurement of Temperature. 



The platinum thermometers were of the standard form, thick platinum leads and 

 compensators connecting the coil with the heads. All connections, both to the 

 thermometers and the bridge, were made by means of small cups hard soldered to the 

 ends of the leads and containing a fusible metal which expanded on solidification. 

 The electrical connection thus formed was a perfect one and easily disconnected and 

 re-made. 



In our earlier experiments two thermometers, labelled AB and CD, were used. The 

 constants of these thermometers have been previously published, and as far as we 

 can detect, show no signs of change over a lapse of 1 5 years. 



Their resistance was ascertained by means of a Callendar and Griffiths " self- 

 testing " bridge,* containing bare Pt-Ag coils immersed in rapidly stirred oil. 



Thus, the temperature of the coils could be ascertained with great accuracy. 



The bridge was carefully calibrated at the beginning of this work and all its coils 

 and bridge wire divisions expressed in terms of the mean box unit ; the absolute value 

 of which, for the purposes of temperature measurements, was of no consequence. 



The slight inequality (but 27 parts in 1,000,000) of the " equal arms" (s, and s 2 ) 

 was ascertained in the usual manner by observations of the apparent alteration in the 

 resistance of a platinum thermometer immersed in ice, caused by exchanging the 

 positions of s, and s.j. 



The resulting correction has been applied to all our measurements of resistance 

 taken with this box. 



All the precautions previously published by one of us were observed, and we do not 



* Hereafter referred to as " Box A." This bridge wan last used in 1900, and it was then observed that 

 one of iU larger coila was showing signs of change. That coil was replaced by another one, and hence a 

 rccalibration of the whole bridge was necessary. 



