THE PROPERTIES OF METALLIC SUBSTANCES 



395 



tungsten or nickel. On the other hand, certain variations occur in the 

 order of the effects. Thus, due to the addition of aluminium, the con- 

 ductance of iron is lowered very nearly as much as due to that of silicon. 

 The resistance-temperature coefficient of solid alloys of the second 

 class likewise varies continuously as a function of composition. The 

 curve of temperature coefficients is similar to the conductance curve, being 

 convex toward the axis of concentration and having a minimum point in 

 the neighborhood of a composition of 50-50. In Figure 69 is shown the 

 curve of temperature coefficients for alloys of silver and gold. It will 



0.004 



I 0*01 



a 



H . 



"i 



90 



100 



A* 



Composition, volume per cent. 

 FIG. 69. Temperature Coefficient of Silver-Gold Alloys as a Function of Composition. 



be observed that the temperature coefficient falls from a value of approxi- 

 mately 4 X 10" 3 for the pure elements to 7.5 X 10~ 4 for an alloy contain- 

 ing 50 volume per cent, each, of the constituents. This behavior of homo- 

 geneous metallic alloys is general. In many cases, the effect is very 

 pronounced and the temperature coefficient falls to very low values. 



With decreasing temperature, particularly at low temperatures, the 

 resistance of homogeneous metallic alloys decreases nearly as a linear 

 function of the temperature. This form of the curve persists even to the 

 lowest temperatures attainable. Apparently, then, the resistance of 

 alloys of this type approaches a finite limiting value at the absolute zero. 

 In the following table are given values of the resistance of manganin wire 

 (84 Cu, 12 Mn, 4 Ni) down to liquid helium temperatures. 



TABLE CLVIII. 

 RESISTANCE OF MANGANIN WIRE AT Low TEMPERATURES. 



Temp ..... 16.5 193.1 201.7 253.3 258.0 269.0 271 5 

 Resist ..... 124.20 119.35 117.90 113.42 112.91 111.92 111.71 



