THE PROPERTIES OF METALLIC SUBSTANCES 389 



As may be seen from the table, expansion of the metal on melting is, in 

 general, accompanied by an increase of resistance. The change in the 

 specific conductance is particularly marked in the case of mercury. In 

 the case of antimony and bismuth, the specific conductance increases on 

 fusion. This is particularly marked in the case of bismuth, which 

 expands on fusion. 



A change in state of an elementary metal is at times accompanied by 

 a discontinuous change in the conductance values and at times only by 

 discontinuity in the temperature coefficient. The transition from gray 

 tin to ordinary tin is doubtless accompanied by a discontinuous change 

 in resistance, although the specific conductance of gray tin appears not 

 to have been determined. In the case of elementary metals of very low 

 conducting power, such as metallic silicon, discontinuous changes in the 

 conductance curve have been observed. In other cases, as, for example, 

 the transition of the magnetic metals at the recalescence point, the resist- 

 ance curve itself is continuous, but the temperature coefficient under- 

 goes a discontinuous change, as we shall see below. 



4. The Conductance of Elementary Metals as a Function of Tem- 

 perature. The electrical properties of different solid elementary metals 

 are strikingly similar. With increasing temperature, the resistance of 

 elementary metals increases, the mean coefficient having a value in the 

 neighborhood of 0.004, which does not differ greatly from the coefficient 

 of expansion of gases at low pressures. Certain metals, as, for example, 

 the magnetic metals iron and nickel, have coefficients much higher than 

 this value, particularly at higher temperatures. The resistance of most 

 metals increases approximately as a linear function of the temperature, 

 and over larger temperature ranges the resistance may be expressed very 

 nearly as a function of the temperature by means of a quadratic 

 equation. 



With decreasing temperature, the resistance of pure metals decreases 

 and, down to liquid air temperatures, it would appear that a value of 

 zero is being approached as a limit at the absolute zero. The experi- 

 ments of Kammerlingh Onnes at liquid helium temperatures, however, 

 have brought to light the remarkable fact that at very low temperatures 

 the resistance of pure metals undergoes a discontinuous change. When 

 a certain temperature is reached, the resistance falls off abruptly to 

 values which are almost negligible, if not actually zero. 8 For example, 

 at 4.24 K. the resistance of mercury in terms of its value at (extrapo- 



Kammerlingh Onnes, numerous papers in the Proceedings of the KoninklHkP Ak*/i 

 emie van Wetenschaf ten te Amsterdam. A summary of the work relatfng ; t "the pr 

 of metals at low temperatures will be found in articles by J. 



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