" Direct-reading " Platinum Thermometer 

 t in terms of n. — From (3) and (5), 



10*7406 n-684-9 





11-677-0-0109 ?i 



That is, 



n = 



11-677 f + 684-9 

 10-7406 + 0-01093 £ 



69 



(6) 

 (?) 



From (7) the plank may be calibrated " directly/' in tem- 

 perature degrees. It will, however, be more convenient to 

 plot a curve co-ordinating n and t. Such a curve does not 

 greatly differ from a straight line ; it is therefore sufficiently 

 accurate to calculate n for every successive five degrees 

 within the range. 



Accuracy attained. 



Mr. E. H. Griffiths has been good enough to examine my 

 figures, and he agrees that, if the original standardization of the 

 platinum coils is carefully attended to, this apparatus may be 

 relied upon to measure temperature with an accuracy of one 

 tenth of a Fahrenheit degree. Unfortunately, the platinum 

 coils were sealed hard and fast within their respective dielec- 

 trics before this slide-bridge method was adopted ; so that it 

 has not been possible to check the readings in a liquid, as 

 against a mercurial thermometer. This is very much to be 

 regretted. By taking the mean of the readings of mercurial 

 thermometers in the tank, and the readings of the mercurial 

 thermometers in the tubes within the dielectrics, and com- 

 paring this mean with the value of t derived from (6), it is 

 possible to make a very rough comparison. It should be 

 carefully noticed, however, that " tube " temperatures include 

 all the errors which it is the very object of the present 

 method to avoid. The following table must be regarded with 

 that limitation. 



Table I. 



Date. 



Tube- 

 temperature. 



n. 



t, from (6). 



Air- 

 temperature. 



Aug. 22 



760 F. 



135-8 



75-9 F. 



o 



81 F. 



„ 23 



72-5 



132-9 



72*6 



77 



„ 28 



68-3 



128-9 



68-1 



73 



„ 29 



80-5 



139-75 



80-4 



76 



„ 30 



90-6 



147-7 



89-6 



76 



Sept. 2 



1010 



15664 



1001 



78 



„ 3 



110-1 



164-8 



1099 



79 



