Thermometer Screens. 
173 
1919-20.] 
there is any radiation, the air passing the thermometer bulb is very variable 
as regards temperature, rising and falling quickly, changes of 1° F. being 
frequently observed to occur in the thermometer during five seconds. A 
thermometer with an ordinary-sized bulb similarly placed responds very 
slowly to these fluctuations of temperature in the air. Even the fine-bulb 
thermometer does not, in fact, show the extreme variations of temperature 
in the air. Sensitive though it is, it has what might be called a “ thermal 
inertia ” * of appreciable amount. If the thermometer were smaller and of 
less thermal inertia, its up-and-down movements would be greater. Probably 
an electric resistance thermometer with a very fine short wire would 
respond more quickly to the variations of air temperature. Every thermo- 
meter will have its own thermal inertia determining how quickly it will 
respond to, and how far it will follow after, a temperature change in the 
air. With solar radiation heating all kinds of matter at the earth’s 
surface, the air is full of heated currents rising and spreading more or less 
irregularly. What are we then to understand by the temperature of the 
air at a given time and place, and how are we to measure it ? It is obviously 
impossible to measure the temperature of the hottest whiff of air; fora 
registering thermometer of no thermal inertia is unthinkable. Every 
thermometer must give its own averaging of the varying temperatures to 
which it responds more or less sluggishly according to its own thermal 
inertia. 
For example, let the fine-bulb thermometer and an ordinary thermometer 
with a bulb of 5 or 6 mm. diameter be exposed under a sunshade and let 
their indications be observed for a few minutes. While the fine-bulb 
readings are subject to continual ups and downs, the other responds much 
more slowly, neither going so high nor falling so low. If the temperature 
is rising on the whole through a series of variations, the average of the fine- 
bulb readings will be above the average of the other ; and if the tempera- 
ture is falling as a whole, the fine-bulb thermometer will give the lower 
average. (See paper on “ Thermometer Screens,” part iv, Proc. Roy. Soc. 
Edin., vol. xiv, 1886-87, for detailed experiments.) 
Thus there is nothing really determinate until we fix on a definite size 
of bulb for the thermometer, and make the thermal inertia of the screen as 
small as possible. If the thermal inertia of thermometer and screen is very 
small, the thermometer will give nearly the average temperature of a small 
* [Dr Aitken introduced the word “ inertia ” to mean the slowness of response of any 
body to a heating or cooling process. It depends on many factors, such as thermal capacity, 
absorption, radiation, conductivity, etc. I have taken the liberty of prefixing the word 
“ thermal” in all cases, so as to prevent confusion. — C. G. K.] 
