.~.ti I>1'. ( '. Cliri-r. ////. ;////'////,< OH 



other thermometers, has to l>e multiplied by 0'4 to render it comparable 

 as an interval of temperature. 



52. Particulars of the degree of accuracy attainable in determining 

 pt, may also be of interest. In this case, of course, the uncertainty 

 existing as to the true law of variation of the boiling point of sulphur 

 with pressure should be lx>rne in mind. Taking all the observations 

 made with K-_>, Ka, K 4 , Kj, and K 7 , excluding one with K 4 and the first 

 three with K 7 as probably faulty, and employing the Callendar-Gritiiths 

 formula (10), I find for the average departure of individual values of pt t 

 from their arithmetic mean the following results : 



Table XXIV. Average Departure of Values of pt g from their 

 Arithmetic Mean. 



The substitution of formula (17) for the Callendar-Griffiths formula 

 reduces the average departures in the case of K 3 , K 4 , and K 7 to 0'07, 

 0*05, and 0'05 respectively, but increases the mean departures in 

 K > and K 5 . 



Roughly speaking, the average departure from the mean in the case 

 of pt t is nearly ten times as large as in the case of the fundamental 

 interval. This, however, may partly arise from the fact that sulphur 

 point observations were frequently taken on the day subsequent to that 

 on which was determined the value of RO used in calculating pt t . 



Improvements suggested in Apparatus. 



53. The experiments have suggested various desiderata, which it 

 may be well to summarise. 



1. It is desirable that some simple and more perfect method should 



be devised for eliminating the effects of changes in the relative 

 resistance of the leads and the proportional arms. 



2. The temperature of the resistance coils and bridge wire should be 



exposed to much less uncertainty than in the Kew box, or else 

 these resistances should be made of some material with a much 

 smaller temperature coefficient than platinum silver. 



3. The bridge-wire scale is too much contracted for very exact work 



with thermometers of such low resistance as the ordinary Kew 

 ones; it is desirable, if accuracy to 0'001 C. is aimed at, that 

 1 cm. of bridge wire should not answer to more than 0*1 C. 



