MEASUREMENT OP TEMPERATURE. 
179 
Corrections to be applied to the Air Thermometer. 
Air does not follow exactly the equation of elasticity of a perfect gas pvj6 = mk. 
Since, however, it follows this law very approximately, its true equation of elasticity 
may be written 
j = mk( 1 —(f)), 
where </> is an unknown function of the variables which must be very small. The 
experiments bearing on this deviation are those of Regnault on the compressibility 
of gases at different temperatures and the comparison of air thermometers ; and, in 
particular, the experiment of Thomson and Joule (‘Phil. Trans./ 1862) “On some 
Thermal Effects of Fluids in Motion.” 
From all these experiments Rankine deduced as the approximate equation of 
elasticity v = R-— -§-ACp^- 
It may, of course, be written in our notation in the form 
- Q = 
(E) 
in which for air 
*=+Doim |. (§>)*. 
Table. d>. 
The following Table gives approximately the values of this correction at different 
points for the standard air-thermometer 
At constant density. 
At constant pressure. 
Formula. 
P~Po 
v-v 0 
t = - 100 . 
o — „ _ 1 UO. 
Pl-Po 
True 
temperature 
Centigiade. 
Values of <p. 
p 0 =76 centims. 
Correction to 
true 
temperature. 
Values of 0 . 
p = 76 centims. 
Correction to 
true 
temperature. 
0 
•001173 
+ •00 
•001173 
+ -oo 
100 
•000626 
+ •00 
•000457 
+ -oo 
200 
•000393 
+ •04 
•000225 
+ -084 
300 
■000267 
+ •09 
•000127 
+ -20 
500 
•000147 
+ •23 
•000052 
+ -47 
1000 
•000054 
+ •62 
•000012 
+ 119 
* This is the form and notation given by Clausius, ‘Mechanical Theory of Heat,’ p. 227. 
2 A 2 
