100 Profs. Dewar and Fleming on the 



meter P x . These platinum temperatures it may be mentioned 

 are defined as follows : — A platinum wire has its resistance 

 determined at 0° C. and 100° C, correction being made for 

 the resistance of its own leading-in wires. A horizontal 

 line on a chart is then taken to represent temperature on the 

 platinum scale and ordinates are set up at distances represented 

 by 100 divisions on this horizontal line, and these ordinates 

 made proportional in length to the true resistance R and Rioo 

 of the standard platinum wire at 0° C. and 100° 0. respec- 

 tively. Through the summit of these ordinates a straight 

 line is then drawn, and the point where it cuts the horizontal 

 axis of temperature is the absolute zero of temperature by 

 that wire. The platinum temperature of any other body, say 

 of boiling liquid oxygen, is then obtained by measuring the 

 resistance of the standard platinum wire immersed in this 

 medium, and after correcting for the resistance of the leading- 

 in wires, obtaining the true resistance R of the standard wire 

 in the liquid oxygen. On looking out on the chart the vertical 

 ordinate, which to the selected scale represents this resistance, 

 it will indicate by its abscissa the platinum temperature pt 

 by that thermometer of the boiling liquid oxygen. For our 

 standard thermometer P : the absolute zero is — 283 o, plati- 

 num degrees on the above-described scale of temperature. 



The coefficients and absolute zero-points of the 

 thermometers ~P 1 and P 2 are as follows : — 

 The constants of our standard platinum thermometer P x are: — 



R = resistance of platinum wire at 0° C. =3*1059 ohms. 

 R 100 = resistance of platinum wire at 100° C. = 4'2034 ohms. 

 R = resistance at any platinum temperature pt. 

 pt = platinum temperature as defined by the equation 



pt= 100 £=\. 



The resistance R at any platinum temperature as defined 

 by this thermometer is given by the equation 



R = -010975 (p*+ 283-0). 



a = mean temperature-coefficient between 0° C. and 100° C. 

 = •003534. 



Rioo — Rq 



a " 100 R ' 



—- = reciprocal of temperature-coefficient = — 283 o, 0. 



The absolute zero of this wire is — 283°*0 in terms of its own 

 platinum temperature. 



