332 Profs. Dewar and Fleming on Electrical Resistance 



to 



— 200° C. Accordingly the distinction between these two 

 metals, in this respect, extends over a very large range of 

 temperature. 



The most interesting fact which these experiments have 

 brought out is the enormous decrease in specific resistance 

 experienced by the perfectly pure metals when cooled to these 

 low temperatures. Thus the electrical resistance of a given 

 pure iron wire at —197° C. is only one twenty-third part of 

 that which it is at + 100° 0. In the case of pure copper 

 the ratio of resistance is about one to eleven for the same 

 change of temperature. The very smallest impurity greatly 

 affects this decrease. In the case of some nickel wire sup- 

 posed to be pure we found that the specific resistance at 

 zero Centigrade was 13387 electromagnetic units, and that 

 the specific resistance at —182° C. was 6737. On repeating 

 the measurement, however, with some absolutely pure nickel, 

 obtained by Mr. Ludwig Mond's process of depositing the 

 metal on glass by heating the gaseous compound of nickel 

 and carbonic oxide, we found a very different result. In 

 this last case, although the specific resistance at zero Centi- 

 grade was not very different, viz. about 12000 units, the 

 specific resistance at —182° C. was only 1900, showing a 

 far greater decrease. For the perfectly pure metals, there- 

 fore, it seems probable that as the temperature is lowered 

 towards the absolute zero the specific electrical resistance 

 decreases so that it either vanishes at the absolute zero or 

 reaches a very small residual value. Clausius made the sug- 

 gestion in 1858 (Fogg. Ann. vol. civ. p. 650) that the elec- 

 trical resistance of all pure metals is proportional to the 

 absolute temperature. Owing to the marked curvature of the 

 resistance-temperature lines this statement is only very ap- 

 proximately true for a few metals, and not at all for others, 

 but it yet remains not improbable that the electrical resist- 

 ance of all pure metals would at the absolute zero be either 

 null or exceedingly small. 



§ 5. In the course of our experiments we found that the 

 trend of the curve of specific resistance drawn with absolute 

 temperatures as abscissae seemed to give a very good indication 

 of the chemical purity of the metal. If that curve tended 

 downward so as to indicate that if prolonged it would pro- 

 bably pass through the absolute zero, the metal was indicated 

 as pure. If, however, as in the case of the palladium wire and 

 the first nickel wire used, it did not so tend, then impurity 

 was probably in some way present. 



We next directed our attention to alloys, and the following 

 is a Table of the results obtained for them, treated in the same 

 manner as regards temperature : — 



