Comparisons of Mercury and Platinum Thermometers. 5 



on the previous history of the wire. The development of this 

 type of thermometer was next taken up by Oallendar, who, in 

 an exhaustive series of researches (Phil. Trans. Roy. Soc, 

 vol. clxxviii. A), showed that if the platinum wire is fairly pure 

 to begin with, and if it has been well annealed, the resistance 

 is always the same at the same temperature, even if the wire 

 has been repeatedly exposed to quite high temperatures in the 

 interval. In the light of these researches, it is plain that the 

 failure of the Siemens' resistance pyrometer was simply due 

 to faulty construction, which exposed the wire to the action 

 of injurious vapours &c. from the clay on which it was wound 

 and the iron cylinder in which it was enclosed. 



The extensive researches of Dewar and Fleming (Phil. 

 Mag. 1893) and Cailletet and Colardeau (J. de Phys. t. viii. 

 1888) at very low temperatures, and those of Heycock and 

 Neville (Chem. Soc. Trans. 1895) at very high temperatures, 

 taken with those of Oallendar and Griffiths (Phil. Trans. Roy. 

 Soc. vol. clxxviii. A, 1887; vol. clxxxii. A, 1891) at ordinary 

 temperatures, establish beyond a doubt the remarkable ac- 

 curacy and constancy of this instrument over a very wide 

 range of temperature. 



Temperature on the platinum scale is denned by the 

 equation 



where pt denotes the platinum temperature, R the resistance 

 of the platinum coil of the thermometer at that temperature, 

 R its resistance at 0° C, and R x the resistance at 100° C. 

 From the above definition it is seen that platinum temperature 

 is independent of the unit of resistance employed. 



Oallendar and Griffiths, in an elaborate comparison of the 

 platinum and air thermometers (Phil. Trans. Roy. Soc. 

 vol. clxxxii. A, 1891), have shown that the relation between 

 the two scales of temperature can be expressed by an equation 

 of the form 



j_y ±\ 



aoo/ ~"ioo y 



where t is the temperature on the air-scale, pt the platinum 

 tempjerature as defined above, and 8 a constant depending on 

 the specimen of wire used. This relation being a parabolic 

 one, only three temperatures are necessary for the complete 

 standardization of a platinum thermometer, i. e., for the 

 determination of 5. Two temperatures always used for this 

 purpose are that of melting ice, 0° 0., and that of water 

 boiling freely under a pressure of 760 mm. of mercury whose 



>-*=«{(; 



