958 



Transactions of tee American Institute. 



Example. 



Suppose we take as a standard a copper wire, ten feet long, No. 26 

 by the American gauge (Darling, Brown & Sharp), whose diameter 

 by the following table is sixteen mils (thousandths of an inch), and 

 find its resistance to be .44 ohms, and another wire of same length, 

 No. 30, whose diameter is ten mils ; the square of the latter is 100, 

 and that of the former 256. 



256 x .44 



100 



=1.13. 



1.13 ohms, therefore, should be the resistance of the No. 30 wire, if 

 the specific resistance of the metals of which the two wires are com- 

 posed are equal ; but on trial we find its resistance to be 1.9 ohms. 



Assuming 100 as the specific resistance of the standard metal, 

 1.13 : 100:: 1.9 : 168; 

 the specific resistance of the tested metal is, therefore, sixty-eight per 

 cent greater than the standard, i. <?., 168 to 100. 



Brown & Sharp's sheet metal gauge, which determines the diameter 

 of a wire to the y^Vjj- of an inch, is the best measure for this purpose. 



Table of Diameters of Wires expressed in mils {tJwusandths of an 



inch). 



40. Or the following method may sometimes be more available, 

 and is more exact : 



