1887.] J. T. Bottomley on a Tractical Air Thermometer. 85 
Friday, January 6, 1888. 
Sm WILLIAM THOMSON, F.R.S., President, in the Chair. 
The following Communications were read : — 
1. On a Practical Constant- Volume Air Thermometer. 
By J. T. Bottomley, Esq. 
In the fourth Memoire of his celebrated Relation des Experiences, 
published in 1847, Begnaulb gives cogent reasons for preferring the 
air thermometer before any other as the instrument by means of 
which temperatures may be defined, and high temperatures deter- 
mined. The thermodynamic researches of Sir William Thomson have 
furnished an absolute thermodynamic definition of temperatures ] and 
the experimental researches of Dr Joule and Sir William Thomson 
have established the practical agreement of Eegnault’s air ther- 
mometers with the thermodynamic scale of temperatures. Lastly, 
the air thermometer is at present the only instrument, with the excep- 
tion of a mercurial thermometer which has been compared with an 
air thermometer, by means of which temperatures higher than, say, 
150° C. or 200° C. can be determined within 3° C. or 4° 
In experimenting on the resistance of platinum and carbon fila- 
ments at high temperatures, in connection with a research on ther- 
mal radiation with which I have been engaged, I have used air 
thermometers of various forms ; and I have recently been using a 
constant-volume air thermometer, which I first described to Pro- 
fessor Gray of University College, Bangor, just two years ago 
(January 1886), and'partially constructed for him at that time. It 
is this instrument, greatly improved as to practical details, which I 
now desire to bring before the Koyal Society. 
The best known constant-volume air thermometer is that of Jolly 
of Vienna. It is a convenient instrument, and is fairly accurate for 
moderate temperatures ; but for high temperatures a correction, 
which it is necessary to apply on account of expulsion of air from 
the heated part of the thermometer, becomes serious, at any rate 
* Mr H. L. Calleiidar has proposed to use the resistance of platinum for 
thermometric purposes ; hut in this case also the final standard of reference is 
the air thermometer. 
