II. A Comparison of Platinum and Gas Thermometers, including a Determination of 

 (lie /foiling- Point of Sulphur on the Nitrogen Scale. An Account of Experiments 

 made in the Laboratory of the Bureau International des Puids et Mesures at 

 Sevres. 



/}>/ JOHN AI.I.KN HAIJKKK, I).X<:, Formerly Bishop Berkeley Fellow of Owens College, 

 Manchester, Research Assistant at Kew Observatory, and PIERRE CHAPPUIS, 

 Ph.D., Savant Attache au Bureau International des Poids et Mesures, Sevret. 



Communicated by the Kew Observatory Committee. 



Received and Read June 15, 1899. 



[PLATE 1.] 



CONTENTS. 



Page. 



I. Introduction and historical re'sum^ 38 



II. Objects of the work 40 



III. to XII. Discussion of different forms of platinum thermometer apparatus and 



construction of new resistance-bridge and accessories 41 



XIII. Galvanometer 49 



XIV. and XV. Platinum thermometers 51 



XVI. Standardization of platinum thermometer apparatus 52 



XVII. Calibration of the bridge-wire 52 



XVIII. Determination of the coil-values 55 



XIX. Progressive changes in coil-values 58 



XX. Determination of the temperature-coefficient of the coils 59 



XXI. Fixed points of the platinum thermometers . 61 



XXII. Heating effect of battery-current on the thermometer wire 61 



XXIII. Determination of the centre of the bridge .... 62 



XXIV. General considerations on the gas thermometer 63 



XXV. Comparison of platinum thermometers with mercury standards 64 



XXVI. Description of the gas thermometer 67 



. Reservoir 67 



b. and r. Barometer and manometer 69 



d. Measurement of pressure 70 



e. Divided scale 71 



XXVII. Zero apparatus ' 71 



XX\ III. Steam-point apparatus 72 



XXIX. Comparison-bath for range 80* to 200' 73 



VOL. CXCIV. A 253. 1S.3.1900. 



