1905.] On the Platinum and Normal Scales of Temperature. 531 



Date and place. 

 July, 1903, London .. 

 October, 1903, London 

 July, 1904, Bristol 



Ice point Steam point 



measured. measured. o. 



Three times Twice 0-0038989 



Once 0-0038977 



Twice 0-0038988 



The probable value of the coefficient is, therefore, about 0-003899. 

 It may be pointed out that the maximum variation in the ice point 

 would correspond to one unit in the next place of decimals. The 

 variations in the experimental values of a are entirely attributable to 

 errors in the determination of the steam point. In any case an error 

 of one unit in the last place of decimals is insignificant and amounts 

 only to an error of : 



0-005 at -}- 33 T 



0-05 



- 200 T, 



Comparison of the Platinum and Hydrogen Thermometers in 

 Air. The hydrogen thermometer, employed for this purpose, has 

 already been described by one of us in conjunction with Drs. Senter and 

 Jaquerod.* The method of working with and the degree of accuracy 

 of the readings have been discussed in detail, and it is only necessary 

 to state that the maximum errors never exceed 0-05 at - 190 C. 

 At the ice point the pressure of the gas in the thermometer was about 

 1000 mm. 



The platinum thermometer was held in a clamp so that the coil 

 was almost in contact with the bulb of the platinum thermometer. 

 The liquid air surrounding them was contained in a cylindrical silvered 

 vacuum-vessel 20 cm. in diameter and 5 cm. deep, which, in the 

 second and third set of experiments, was placed inside a somewhat 

 larger silvered vacuum-vessel. Some liquid air was poured into the 

 space between the two vessels, and both the opening of the annular 

 space and the mouth of the inner vessel were plugged with sheep's 

 wool. 



With this arrangement the rate of rise of temperature as the air 

 evaporated amounted to 0*002 per minute. And under these condi- 

 tions it can safely be assumed that the temperature of neither ther- 

 mometers differed from that of the bath by an appreciable amount. 



It is noteworthy that under such conditions liquid air appears to 

 evaporate from the surface and does not undergo regular fractiona- 

 tion. 



The following are the results of our experiments : 



Phil. Trans.,' A, vol. 200, 1902-3. 



