G08 



Dr. A. Russell on the Convection of Heat 



the assumption that the formulas given above for cooling by 

 incompressible fluids may be applied tor gases, we see that 

 the fusing current varies as the l'25th power of the radius. 



13. Schwartz' s Experimental Results. 



Expressing the fusing current by \a n where \ and n ara 

 constants for a given metal, the following results were 

 obtained for \ and n. 



Metal. 

 Copper (tinned) 



Tin 



Silver 



Aluminium 



Length of fuse. 



5 cms. and upwards .. 



3 8 cms 



7'6 cms. and upwards 

 15 cms. ,, 



12-7 cms. 

 10 cms. „ 



S.W.G. 



Fusing 

 Currents. 



1 to 10 



358 



1-20 



47 to 33 



,, 



„ 



491 



1-26 



43 to 20 



,, 



147 



113 



20 to 7 



10 to 80 



239 



1-32 



35 to 18 



7 to 70 



967 



1-29 



42 to 20 



2 to 30 



640 



1 27 



In the case of most of the wires placing them vertically 

 did not affect the value of n. Before this paper was published 

 electricians, making the assumption that the heat emitted 

 per unit surface of the wire was independent of its radius, 

 deduced that n should be 1*5. 



14. Steady Temperature of a Wire carrying an 

 Electric Current. 



If we assume that the volume resistivity of a wire varies 

 with temperature according to the law 



Pe = Po( l + ae )' 

 we get by (26), 



${ 8\/^^ a -0-239C 2 ^ I =0-239C 2 A, . (2$) 



and thus can be easily computed. The value of C must of 

 course be less than the fusing current. 



Suppose, for example, that the wire is being cooled by a 

 stream of ice-cold water. We shall take 



* = a = 1 and k = 0'OOIG. 



Hence 



m {*«!£-*}' «■■• • -^ 



