250 Electric Resistance Thermometry. " [Feb. 25, 



Fig. l. 



O .... T 



It is clear that" in no case can anything parabolic connect resistances 

 and temperatures ranging from the boiling point of water to that of 

 hydrogen. Just as we seek for a circle of curvature at any point of a 

 curve, so in the present case we may, at a point, or over a short range 

 of the f, seek for an approximate parabola ; but any such parabola 

 will be of no, or little, use for extrapolation. I have mentioned that 

 the value of 8 for my platinum (Callendar) parabola is about 2*5, 

 whereas observers at high temperatures finds its value about 1/5 for 

 pure platinum. Such differences have been found by others also, but 

 they do not seem to have attracted attention. On looking at fig. 2, we 

 shall find the discrepancy easily accounted for. The portion ABCD of 

 the curve represents roughly (on a much reduced scale, in order to 

 show the curvature more clearly) the curve of P 2 r, given in Plate 1. 

 The points B, D, E, represent the boiling points of oxygen, water, and 

 sulphur ; and C, F, represent the freezing points of water and silver. 



Fig. 2, 





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-300° 0° 1000° 



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