338 Sectional Areas for Copper and Iron Lightning-Rods. 



But B= -, where X is the specific resistance of the metal 



per cubic centimetre at its temperature of fusion. 



We may take the melting-point of copper as 1400° C, and 

 that of wrought iron as 2000° 0.*, and, in order to find X, 

 assume that Dr. William Siemens's formula, which he verified 

 to 1000° C., holds goodj, viz. 



\f=\ (0-026577** + 0-0031443*--0-29751) for copper, 



\f=\ (0-072545** + 0'0038133*-l-23971) for iron. 



The temperature t in these formulae is to be measured from 

 the absolute zero; so that we have £ = 1673 for copper, and 

 £ = 2273 for iron. 



The value of X P er cubic centimetre of copper is 1*652 mi- 

 crohm, and per cubic centimetre of iron is 9*827 microhmsj. 



Thus the value of Xt per cubic centimetre of copper becomes 

 about 10 microhms at 1673° C, and per cubic centimetre of 

 iron becomes about 107 microhms at 2273° C. 



Hence 



10 



H= const — for copper,^ 



and 



H= const for iron. 



Therefore 



and 



T= const 



T= const 



0-1013 x°8-9x a' for C< W 

 107 



for iron. 



0*1218 x 7*8 x A 2 

 Now, putting T = temperature of fusion in each case, 



1400= const — g - f° r copper, 



2000 = const — ro— for iron. 



Therefore 



whence 



( A 



-) 



a / 



A 2 

 1400 112-63 



2000 



= 7-112, 



A 8 , 

 -= 3 nearly 



11-09 



* Rankine's Tables. t Bakerian Lecture, 1871. 



t Jenkin's Cantor Lectures, from Mathiessen's experiments. 



