520 Prof. A. W. Porter and Mr. E. R. Martin 



the 



change to unity the value corresponding to a single coat. 

 This is merely equivalent to changing the standard to one- 

 which is more convenient for calculation. Several observa- 

 tions were made for each thickness ; in the following table 

 the mean values are alone tabulated : — 



1 

 No. of Coats. 



! 



Outside 

 radius. 



Katio of condensations 

 Thick coat. 

 Thin coat. 



R 



Ditto 

 Reduced standard. 



i i 



3 

 4 



6 



8 

 10 

 12 

 14 



18 



•234 cms. 

 •275 

 •318 

 •350 

 •397 

 •447 

 •514 

 •580 

 •671 

 •749 

 •890 

 1-194 



•953 

 •984 

 1-068 

 1-084 

 1-099 

 1-155 

 1-234 

 1-211 

 1-180 

 1185 

 1161 

 1070 



rooo 



1032 

 1122 

 1140 

 1-154 

 1-214 

 1-296 

 1-272 

 1-239 

 1-244 

 1-217 

 1-125 



The theoretic value for the ratio should be (assuming that 

 the temperature at the external surface of the brass may ba 

 taken as constant throughout the experiments) 



1 



•234E 



+ 



1 

 k 



log* 



•234 

 •19 



1 

 bE 



+ 



1 



h 



log e 



b 

 •19 



Ratio = R = 



where *19 is the radius of the tube uncoated, 



'234 „ v with one coat, 



E and h are emissivity and thermal conductivity of 

 asbestos. 



b = external radius of coated tube. 



Now E and h both depend upon the excess temperature, 



and E depends also upon the radius. The experimental 



results are not accurate enough, however, to justify one in 



trying to take these variations into account. Assuming 



constancy in these data, it is easy to show that R should 



k 

 be a maximum when I— =. Now from the diagram the 



maximum is found to occur when b = "57 cm. (about). The 

 position of this maximum fixes the ratio of k to E. Putting 

 the value of k derived from this value of b into the equation, 



