390 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



lated) is 0.163, while at 4.185 the resistance is less than 10~ 6 , and at 

 2.45 less than 2 X 10~ 10 . Similarly, the resistance of tin vanishes at a 

 temperature of 3.78 K. and that of thallium at 2.3 K. The resistance of 

 lead vanishes at a temperature between 4.3 and 20, probably in the 

 neighborhood of 6 K. Metals in a condition in which their resistance 

 vanishes are said to be in a supraconducting state. Certain metals, such 

 as platinum and copper, do not exhibit supraconductance. In such 

 metals the conductance falls to a low limiting value, after which it 

 remains independent of temperature. In the following table are given 

 values for the resistance of platinum in arbitrary units, at a series of 

 temperatures. 



TABLE CLV. 

 RESISTANCE OF PLATINUM AT Low TEMPERATURES. 



T abs. Resistance 



273.1 1.0 



20.1 0.0170 



14.3 0.0136 



4.3 0.0119 



2.3 0.0119 



1.5 0.0119 



It is apparent from this table that at a temperature in the neighborhood 

 of 4.3 absolute the resistance of platinum falls to a value a little greater 

 than 0.01 of its value at 0. Below this temperature, the resistance 

 remains constant. Similar results have been obtained for other metals 

 such as copper and iron. Apparently, those metals, which exhibit a 

 marked tendency to form solid solutions with other metals, do not 

 exhibit the phenomenon of supraconductance. It has been suggested 

 that the absence of this phenomenon in these metals is due to the influ- 

 ence of minute traces of impurities. 



The significance of the phenomenon of supraconductance is not fully 

 understood as yet. Various theories have been proposed in explanation 

 of this phenomenon, as, for example, that of J. J. Thomson. 9 Bridg- 

 man 10 has recently suggested that a polymorphic change takes place at 

 the point where supraconductance intervenes. According to this view, 

 the normal state of a substance, or of a crystal, at very low temperatures 

 is that of supraconductance. The residual resistance found in the case 

 of such metals as platinum is due to non-homogeneity between the sur- 

 faces of the individual crystals of which the conductor is composed. At 



J. J. Thomson, Phil. Mag. SO, 192 (1915). 

 10 Bridgman, J. Wash. Acad. 11, 455. 



