170 THE REALITIES OF MODERN SCIENCE 



sure per degree represented by a is found by dividing 

 by PO , thus 



_ PIOQ PO so\ 



- 100 Po 



Careful measurements have shown the numerical value 

 of a, the " pressure coefficient at constant volume/' 

 to depend somewhat upon the kind of gas. Hydrogen 

 has therefore been adopted as the standard for the gas 

 thermometer. 



Returning to equation (2) we remember that the 



2 ( mv 2 \ 

 - -- 



quantity -N( -^- } is constant if the temperature is 

 o \ 2i J 



constant and that we decided to measure temperature 

 from an absolute zero corresponding to a value of 

 Nmv 2 /2 equal to zero. We are therefore interested 

 in finding where our arbitrary reference point of zero 

 degrees Centigrade may happen to be located in this 

 absolute scale of temperatures. 



Let us represent temperatures measured from this 

 absolute zero by T. Since we do not as yet know the 

 absolute temperature corresponding to C. let us 

 represent it by T . In terms of absolute temperature 

 a temperature of t C. is expressed as t = TT , since 

 T= T +t. The value of the pressure, P t , corresponding 

 to t is expressed in terms of the pressure coefficient and 

 the pressure at C., namely P Q , as follows: 



P, = P (1+0 (4) 



Substituting for t, and for a its value for hydrogen of 

 0.00367 = 1/273 gives 



