234 



Mr. H. L. Callendar. 







preferable to use t= =^-100, and to call the temperature so 



j&y — R 



calculated temperature by platinum wire, as this involves no assump- 

 tion and different wires nearly agree. It is from a thermometric point 

 of view that the method above described of comparing different 

 wires with the standard is most valuable : but it might also be em- 

 ployed to form very accurate comparative tables of the change of 

 resistance with temperature for different metals referred to platinum 

 as a standard. Such tables would, however, be of little use apart 

 from the actual specimens of metal employed, except in so far as they 

 tended to elucidate the general phenomenon of the variation of 

 resistance with temperature. With this view a comparison was made 

 hetween platinum and iron, the resistance variation of which metal 

 appeared likely from previous observations to differ widely from 

 that of platinum. The curve deduced resembled the steam pressure 

 curve, and suggested the formula* 



A formula of the same type was found to satisfy the air thermometer 

 observations for platinum better than any other. The corresponding 



differential equation is tf 3 — (logR)~ 1 = 0. If R l5 R 2 be any two 



CbU Cbv 



observed resistances of the platinum coil, and dashed letters express 

 corresponding quantities at the same temperatures for iron, we have 



from (e), " =^zS-=^— — a by equating tempe- 



log R x log R i log e log R 2 log R 2 



ratures. The comparison observations are tested and found to satisfy 



this equation of condition ; from which we may deduce the ratios 



a : a! : (J3— /3'). Assuming the values of a and (3 for platinum to be 



a =0'0034259, ^3=0-0015290, we find for iron the values a'=0-0045657, 



/3'=0-0007767. 



The following table containing some of the observations taken by 

 this method, with the temperature varying, shows the capabilities of 

 the method and the sensitiveness of the platinum spiral. 



The time observations show the rate of variation of the tempra- 



R 



ture. The resistance calculated by formula (a) — =1 -f 0"003460£, 



R 



shows the extent of the deviation from the straight line. 



The difference column shows how nearly the observations agree 

 with formula (e), and the smallness of the probable error of a single 

 observation. 



* Eegnault, ' Paris, Acad. Sci. Mem.,' vol. 21, 1847, p. 619. 



