1896.] 



oa Electric Eesearch at Loio Temperatures. 



245 



proportional to the resistance of the platinum wire at 0° C. and at 

 100° respectively, and through the tops of these perpendiculars a 

 sloping straight line is drawn until it cuts the axis of temperature at 

 E. The graduation of the horizontal line is continued in both 

 directions on the same scale as the subdivision of the line between 

 the points marked and 100. To measure and define any other 

 temperature, say, for instance, the boiling-point of liquid oxygen 

 under a pressure of 760 mm., we have simply to measure the resist- 

 ance of the platinum wire in the liquid oxygen. We then look out 

 on the chart the ordinate which has the same numerical value as the 

 resistance of the wire in the oxygen, and at the foot of that ordinate 



Fig. 5. 

 Method of constructing a scale of platinum temperature, 



we find a number, viz. ( — 197), which is the temperature of the liquid 

 oxygen on this platinum scale. 



Two questions then arise — first, Do all annealed platinum wires 

 give, when used in this way, the same numerical values for definite 

 and identical temperatures ? The answer to this is, Nearly, but not 

 quite. In the case of two thermometers much used by us, the differ- 

 ence was about half a degree at — 100° C, the boiling-point of liquid 

 ethylene. Into this matter it is not possible here to enter more fully ; 

 suffice it to say that we have invariably referred our temperature 

 measurements to one standard thermometer. The second question is 

 equally important — it is, What is the relation of the scale of tempera- 

 ture so defined to the absolute thermodynamic scale; or, which is 

 very nearly the same thing, to the scale of temperature defined by a 

 constant pressure hydrogen thermometer ? If the air thermometer 

 and platinum thermometer readings are made to agree at 0° C. and 

 100° C, then a temperature which would be called 50° on the 

 Centigrade scale would be denoted by 50*4 nearly on the platinum 

 thermometer spale, and corresponding to - 78° on the Centigrade 

 scale, which is the temperature of carbonic acid melting in ether. 



