248 Prof. J. Dewar. Mectric Resistance Thermometry [Feb. 25, 



freezes about 5° below its boiling point. In the present experiments I 

 have been able to get eight observations in liquid hydrogen boiling 

 under pressures varying from 5 — 50 mm., and it will not lead us 

 appreciably astray to take the temperatures of these observations 

 as (say) 4° below the boiling point. If the law connecting resistance 

 with temperature be parabolic, the very gentle curvature at the boiling- 

 point of hydrogen will allow us to consider the rate of drop in 

 resistance per degree of temperature for the 4° below the boiling- 

 point of hydrogen as roughly the same as that between the boiling- 

 points of oxygen and hydrogen (70°), so that the ratio of these two 

 drops on this supposition should be about 4 : 70, or say one-eighteenth. 

 These ratios are given in Table II. 



Table II. 





P 27 . 



Pt-Rh 29 . 



A %3- 



Au 40 . 



Ag.,3- 



Pd 46 . 



Cu 47 . 



015 



1 269 



0-121 

 6 -836 



0-06 

 3-21 



0-015 

 -495 



0-083 

 2-999 



0- 018 



1- 425 



0-057 

 1 -583 



-006 



1 -512 



1 



or — 

 84 



1 



57 



1 



53 



1 



33 



1 



36 



1 



79 



1 



28 



1 



252 



Now these ratios are all much smaller than one-eighteenth, hence we 

 infer that the curves have taken a more or less quick turn in the 

 neighbourhood of the boiling point of hydrogen, or perhaps above it. 



On Plate 10 the observed resistances are displaj^ed graphically. For 

 convenience I make seven groups, namely platinum, gold, silver, 

 copper, palladium, magnetic metals, and alloys ; and in order to bring 

 the characteristics of these groups into comparison, each metal is 

 supposed to have the resistance of 30 ohms at the freezing point. This 

 number was chosen to suit the scale — the intention being that, roughly, 

 the "plot" of resistance and temperature should be a line equally 

 inclined to the two axes of resistance and temperature. This has been 

 accomplished by taking a centimetre to represent 20° C. in temperature, 

 and 2 ohms in resistance. The diagram for each group has the reading 

 at 0° C. placed 2| cm. higher than that of the group below it, in order 

 to obviate confusion among so many approximately coincident lines. 

 Attention paid to this will enable each group of curves to be clearly 

 seen and compared with the others. 



The first noticeable peculiarity is the close coincidence of the two 

 silver curves. For them, Callendar's a's, the 12's, and the ratios of the 

 resistances at 0° to that at the boiling point of hydrogen are almost the 

 same, although the S's differ much. In like manner the Dickson 

 constants for Ag 43 are all in the same ratio (about 5 : 2) with those for 



Ag 3 4. 



