.352 DR. J. A. HARKER ON THE HIGH-TEMPERATURE STANDARDS 



IX. The Platinum Thermometers. 



The platinum thermometers chosen for the comparison at high temperatures with 

 the gas thermometer were one of a group of six constructed in 1902 at the laboratory 

 for the British Association and described as BA., in the report of the Electrical 

 Standards Committee for 1903, and the much older one, lettered Ko, which had been 

 employed by CHAPPUIS and HARKER in their work at Sevres. Since the date of the 

 Sevres comparisons in 1898 and 1899 this thermometer had been preserved as a 

 standard and used comparatively seldom. 



Its characteristic dimensions are : 



Length from end of porcelain tube to below the wooden collar, 3 5 '5 ceutims. 

 External diameter of tube, 11 - 5 millims. 



The head was of the old pattern with terminals, the leads being entirely of 

 platinum, '020 inch diameter, and the "bulb" wire, '006 inch diameter. 



Thermometer BA 2 is similar in design to CHAPPUIS' and HARKER'S thermometers 

 K s and K y , illustrated in their paper, the head being arranged to be capable of 

 standing a vacuum for a considerable time. 



Its dimensions were as follows : 



Length of tube to under head = 43'5 centims. 

 Internal diameter of porcelain tube = 1 1/5 millims. 

 Length of mica cross = 40 millims. 

 Thickness of " bulb" wire = '006 inch, 

 "lead" ='020 



Both "bulb" and leads were constructed from the stock of wire ot extra high 

 purity prepared by JOHNSON and MATTHEY for the British Association. Details as 

 to the construction of these British Association thermometers are given in the 

 appendix to the Report of the British Association, 1903. 



X. The Resistance Box for Platinum Thermometry. 



The resistance box used was the one originally described by GRIFFITHS in ' Nature,' 

 November 14, 1895, pp. 39 to 46, which had subsequently been modified by the 

 Cambridge Scientific Instrument Company by the substitution of Doulton-ware plug 

 holders for the usual brass blocks to which the individual resistances were connected. 

 In this form the box was used by CHREE in his investigations described in ' Proc. 

 Hoy. Soc.,' vol. 67, pp. 3-58. 



During the period covered by the present work the coils were standardised in the 

 manner described by HARKER and CHA*PPUIS (loc. cit., p. 52), and great care was 

 taken throughout as to the cleanliness of the plugs and to prevent the accumulation 

 of the black deposit which has a tendency to form in the plug holes. The box unit 

 is very exactly '01 ohm, and the nine coil values are arranged on the binary scale 



