28 



ELEMENTS OF ELECTRICITY AND MAGNETISM. 



the resistance of a wire in ohms when the length of the wire is 

 expressed in feet and the sectional area in circular mils, then the 

 value of k must be the resistance of a wire of the given material 

 one foot long and one circular mil in sectional area ; for example, 

 the resistance of a copper wire one foot long and one circular mil 

 in sectional area is about 10.4 ohms at 20 C. 



TABLE. RESISTIVITIES AND TEMPERATURE COEFFICIENTS. 



a = resistance in ohms of a bar I centimeter long and I square centimeter sec- 

 tional area. 



b = resistance in ohms of a wire I foot long and o.ooi inch in diameter. 



c = temperature coefficient of resistance per degree centigrade (mean value be- 

 tween o C. and 100 C. ). 



*See temperature-resistance curve, Fig. 15. 



t Between 18 C. and 19 C. 



J These values differ greatly with different samples. 



15. Resistivities of alloys. The ordinates of the three curves 

 in Fig. 12 represent the resistivities at a given temperature of 

 alloys of zinc and tin, of silver and gold, and of silver and plati- 

 num, respectively, and the abscissas represent the percentages of 

 the constituent metals. The zinc-tin line, marked Zn + Sn, is 

 sensibly straight ; that is, the change of resistance from pure zinc 

 to pure tin is proportional to the percentage of tin in the alloy. 



