514 



Prof. A. W. Porter on tin 



This second radius, which may be called the neutral radius 

 for the particular bare wire, because for it the total tempe- 

 rature effect of the lagging material is zero, is not independent 

 of the radius of the wire. It is not possible to express it, 

 therefore, quite so simply. It is obvious, however, that it 

 corresponds to the value of b for which the expression 



j- + t log b has the same value as for the un coated wire. If 



then the value of this expression be calculated for various values 

 of b and be plotted against b, then the radius of the bare 

 wire and the corresponding neutral radius are the two values 

 of b for which the ordinates are equal. Corresponding values 

 so read off from a curve are tabulated with sufficient accuracy 

 in Table II. 



Table II. — Para Rubber. 



£ = •0001. 



e = -0001. 



Eadius of bare wire. 



Neutral radius. 



•25 cm. 



10 cm. 



•32 



5-8 



•50 



29 



100 



Critical 1-00 



In order to test these results a thin platinum wire ('02 cm. 

 radius) was coated along a part of its length with glass so 

 that the outside radius was *1 cm. When a current is passed 

 through the wire the uncoated portion may be made nearly 

 white-hot without any sensible glow occurring in the coated 

 portions. This is the nature of the result to be expected 

 even from the above simplified theory, for it gives 



Temperature excess of uncoated wire 1 



Temperature excess of coated wire 



/l 1, bV 



izn^a) 



where 6 c = the critical radius. 



In the above case this ratio is 5 nearly. It should be 

 observed in passing that the logarithmic term scarcely affects 

 the ratio, and this will usually be so when b is small compared 

 with the critical radius. 



Thus if the uncoated is raised to 1600° C. the coated will 

 be only at 330° C. 



