Lagging of Pipes and \Vi 



res. 



513 



The old values of the thermal conductivities of badly 

 conducting materials still given in most text-books are mainly 

 due to Forbes. They have been shown, however, to be com- 

 pletely untrustworthy. The values employed here are those 

 obtained by Dr. C. H. Lees by Sir 0. Lodge's method 

 (Trans. Roy. Soc. A. vol. 183. 1892, p. 481). 



With regard to the emissivity there is more indefiniteness 

 as it depends, when the body is in air, not only upon the 

 nature or" the surface but upon its radius, as was first shown 

 by Peclet. Provided, however, that we do not consider 

 sheaths whose radius is very small, we may take *0003 as a 

 fair value. 



The critical values of b are then as given in the following 

 table : — 



Table I. 



Material. 



Thermal 

 conductivity. 



Critical radius 

 in centimetres. 



Crown glass t -00243 



Slate ' 0047 



Shellac -0006 



Para rubber -00038 



Gutta-percha '00046 



Paper I -00031 



Asbestos paper I -00057 



Cork -00013 



Silk -00022 



Cotton -00055 



Flannel 00023 



* Magnesia (Pattinson's Light Calcined)... | '00016 



8'1 

 16 

 2 



1-3 

 1-5 



1-0 

 1-9 

 •4 

 "7 

 1-8 

 •8 

 •5 



* Hutton & Beard, Faraday Society, July 1905. 



Here we have the somewhat startling result that coating a 

 wire with glass up to 8 cms. radius is more and more detri- 

 mental to the maintenance of a high temperature in the wire 

 the thicker the coat is within this range. For gutta-percha 

 the range is up to 1£ cms., and so on for other substances. 



If the coating has an outer radius greater than the critical 

 one the action begins to reverse. But an examination of the 

 formula shows that the improvement (from the point of view 

 of effectiveness in lagging) is very slow, depending ultimately 

 upon a logarithmic term. The result is that very considerable 

 thicknesses must be attained before the temperature of the 

 wire will become the same as if there were no lagging in it 

 at all. This point is reached when 



1 l n . 1 1 , 



Phil. Mag. S. G. Vol. 20. No. 117. Sept. 1910. 2 M 



