Measurements of Precision in Platinum Thermometry. 549 



At this point it is well to call attention to the bridge 

 described on pp. 557, 558. In this, the resistance of all four 

 arms of the bridge keep constant whatever the temperature of 

 the thermometer may be, and no change of external resistance 

 is necessary. In precision work accidental errors are thus 

 avoided *. 



It is now of interest to calculate the minimum time during 

 which the current must flow through the thermometer before 

 the heating effect can be said to be sufficiently steady for an 

 observation of resistance to be made. 



The wire of the thermometer with which we experimented 

 was 49 cm. long and 00152 cm. (0'006 inch) in diameter. 

 If R is the resistance of the wire, L its length, r its radius, 

 and 6 a small difference of temperature between the wire 

 and its surroundings due to a current z, then li the thermal 

 emissivity of the surface is given by 



h = 0'02Si*R/dhr, 



from which we have calculated the following values of h at 

 the icp point, the steam point, and the sulphur point : 



Li 



Temperature. 



0-00X6 



0° "' 



0-0021 



100° 



0*0037 



445° 



If the maximum increase of temperature due to the 

 current is 6 1 and 6 is the increase of temperature t seconds 

 after completing the circuit, then 



where A is the density qf the platinum wire, and § i.s its 

 specific heat, ]Jsing this equation we have calculated for 

 0° C. the increments of temperature for various values 



* A constant current methocj. was first introduce*} by Professor 

 Callendar in 1891. In this method the platinum thermometer is con- 

 nected on the compensator side of the "box in series with the box coils, 

 and is balanced against a compensated resistance, equal to, or greater 

 than, the resistance of the thermometer at the highest point of the range 

 it is plesired to cover. Phil. Trans. A. vol. ccxii. p. 19 (1912). 



f These values are calculated on the assumption that the thermometer 

 increases in temperature by 0°-016 ; C \017, and ti°-019 at 0°, 100 6 , and 

 445° respectively, due to ' a current of 0*01 ampere (see Tables IV. 

 to VI.). " 



Phil Mag. S. g, Vol. 24. No. 142. Oct. J9J2. 2Q 



