Lagging of Pipes and Wires. 



hit 



observations were made on a nickel wire partly coated with 

 gutta-percha. The results in this case are given in Table V. 



Table V. — Nickel wire and Grutta-percha. 



a='025cm. b = '175 cm. 



Excess temperatures. 



Uncovered. 



Covered. 



Eatio. 



19 



14 



1-36 



31 



18 



1-70 



36 



25 



1-44 



65 



27 



24 



67 



24 



2-7 



77 



42 



1*8 



111 



51 



2-2 



The simple theory would require a ratio of about 6. 



The cause of the discrepancy is to be sought for in the 

 variation of emissivity and thermal conductivity with the 

 temperature, and in the case of emissivity with the radius 

 also. The last cause is in fact exceedingly important in the 

 case of wires so fine as those used in these experiments. The 

 ratio ©a/©i should in fact be more nearly 



than — , where E& is the emissivity for the covering and E a 



is that for the wire. In the case of the platinum wire at 

 about 100° excess E is perhaps as much as 7 times E&, hence 

 a ratio of 15 is at once lowered to about 2. 



The full and satisfactory consideration of the question 

 requires, therefore, a complete knowledge of the variation of 

 the various data with temperature. In the absence of this it 

 scarcely seems worth while to develop the theory much 

 further owing to the increased complication. The following 

 formulation may, however, be found useful in case the 

 question should be found of sufficient practical importance. 



The differential equation to be solved is 



z^__ !_ 



Br ~ 27rr' 



