244 Prof. J. Dewar. Electric Resistance Thermometry [Feb. 25, 



" On Electric Resistance Thermometry at the Temperature of 

 Boiling Hydrogen."* By Professor James Dewar, M.A., 

 LL.D., F.K.S. Received February 25,— Kead March 10, 1904. 



[Plate 10.] 



In my paper entitled " The Boiling Point of Liquid Hydrogen 

 determined by Hydrogen and Helium Gas Thermometers," communi- 

 cated to the Society in 1901, I detailed means taken to determine the 

 temperature in gas thermometer degrees at which liquid hydrogen 

 boils. The experiments clearly proved that the constant-volume 

 hydrogen gas thermometer is reliable under varying conditions, down 

 to a temperature some degrees below the boiling point of hydrogen, 

 20° *5 absolute, on the Centigrade scale. 



In the course of making low temperature determinations, a number 

 of thermometers of different kinds were employed, some being 

 constant- volume gas thermometers, others in the form of thermo- 

 electric junctions, and a set depending on change of electric resistance 

 with temperature. The group of thermometers depending upon the 

 assumed relation between electromotive force and temperature broke 

 down about the temperature of liquid oxygen, while those based upon 

 the ordinary law correlating change of electric resistance with tempera- 

 ture failed for all the metals and alloys somewhere above the boiling 

 point of liquid hydrogen. 



In the first experiments made to ascertain the boiling point of 

 liquid hydrogen under a pressure of 30 to 60 mm. a platinum resist- 

 ance thermometer was used, with the result that only 1° of reduction 

 of temperature in the boiling point was recorded instead of some 

 5° indicated by theory. This result suggested that in all probability 

 the rate of change of resistance with the pressure became gradually 

 smaller under very low temperatures. 



In the Bakerian Lecture I adverted shortly to some results obtained 

 in low temperature resistance thermometry, and gave a tablet of 

 numerical values deduced from experimental observations for some of 

 the more prominent metal resistance thermometers. In the present 

 communication the experimental records of eight additional electric 

 resistance thermometers are given, and the results of the observations 

 on all the resistance thermometers used during the investigations 

 are collected and compared. 



Two facts seem to result from this inquiry, viz. : (1) That the 

 resistance of an unalloyed metal continually diminishes with tempera- 

 ture and in each case appears to approach to a definite asymptotic 



* In continuation of Art. 3 of the .Bakerian Lecture, ' Roy. Soc. Proc.,' vol. 68, 

 p. 360. 



t ' Koy. Soc. Proc.,' vol. 68, p. 363. 



