[ 208 ] 



XXV. On the Influence of the Dimensions of a Body on the 

 Thermal Emission from its Surface. By Alfred W. 

 Porter, B.Sc, Demonstrator of Physics, University 

 College, London* . 



IN discussing the rate at which heat passes outwards from a 

 conducting body into the medium in which it is immersed, 

 it is usually assumed that (for small excesses of temperature of 

 the body above its surroundings) it may be taken as propor- 

 tional to the excess of temperature. This law is followed, as 

 far as is known, if the body is in a vacuum and loses heat 

 simply by radiation between its own and surrounding surfaces. 

 If, however, it is immersed in a medium, such as air, which 

 carries away heat by conduction and convection as well as by 

 radiation, the problem becomes more complicated, and to 

 assume that the whole effect may be treated as radiation only 

 does not give results which are even an approximation to those 

 obtained experimentally. For example, on this assumption 

 the amount passing outwards from unit area of the surface per 

 second per unit excess of temperature (?'. e. the " emissivity ") 

 should be independent of the size of the body. Results ob- 

 tained by Peclet from experiments on cylinders and spheres 

 of different sizes show that this constant depends materially 

 upon the sizes of the bodies experimented upon. Peclet's 

 formulae connecting the rate of emission (exclusive of the 

 radiation effect) with the radius (r) for 0° C. excess are : — 



For a horizontal infinitely long *) 9.^0,'^^ 

 cylinder of brass .... J ^"~ 



■13 



For a sphere 1*774 + 



r 



in which kilogrammes, metres, hours are the units employed. 

 Other formulae are given by him which it is unnecessary to 

 quote here. 



These results do not seem to have attracted much notice, as 

 they are not given in any of the text-books of Physics. They 

 do, however, appear in a book compiled for practical men by 

 Box f, from which the formula for the sphere is quoted, on 



* Communicated by the Physical Society : read January 11, 1895. 



+ • A Practical Treatise on Heat for the use of Engineers and 

 Architects,' by Thomas Box. (London, E. and F. N. Spon, 2nd edition, 

 1876.) This book does not appear to be generally known to physicists ; 

 and, judging from recent references to it, I gather that still less is it 

 realized that the author's data are to a great extent obtained from Peclet, 

 although he states the source to which he is indebted in his preface. 



