﻿Connective Cooling in Plaids. 939 



if represented the temperature difference between the two 

 surfaces. A few experiments would soon test the applic- 

 ability in any doubtful case. 



For any circumstances to which the formula applies, the 

 form of the function F may be obtained from an experiment 

 in which only one of the variables (say 6) is altered. We 

 have some knowledge of its form in certain cases. For 

 cooling of large bodies in free air or in enclosures, largo 

 compared with the size of the cooling body, we may write 



hL/k0az(L 3 dgac/kv) n , 

 where ti is usually about 1/4, but tends to be as great as 1/3 

 for large bodies or very hot ones, and to be much smaller 

 for fine wires only slightly heated, as in the present 

 experiments. Measurements of the conductivity of fluids 

 have been made by methods involving fine wires very 

 slightly heated along the axis o£ a small metal tube : the 

 validity of this method implies that ?i = in this extreme 

 case. 



Since for a given set of circumstances the value of n 

 depends upon the size of the model and upon its temperature 

 excess, it is obvious that the cooling power of a fluid depends 

 upon the circumstances in which it is to be used. Retaining 

 from the above formula only those quantities which relate to 

 the properties of the cooling fluid, we have 



h cc k(ac/kv) n , 



and for the conditions so far studied n varies from to 1/3, 

 so that the relative cooling power of a fluid may vary 

 between the limits 



k and (ack 2 jvy. 



Thus it appears for these conditions that the conductivity 

 of the fluid is the preponderating physical property deter- 

 mining cooling power, This result is probably fairly general, 

 for while it may be possible to devise experimental arrange- 

 ments in which k does not enter, suitable conditions for its 

 omission seem generally unlikely to be realized, for it 

 evidently enters vitally whenever the temperature of a 

 particle of fluid (and thus the vigour of its motion and the 

 amount of heat it absorbs or yields) depends upon the 

 thermal conductivity of the adjacent layers. 



The effect of the physical properties, other than thermal 

 conductivity, appears to be such that high specific heat and 

 high coefficient of expansion have the same degree of 

 importance in facilitating cooling that high viscosity has in 

 restricting it. 



3 P 2 



