330 Profs. Dewar and Fleming on Electrical Resistance 



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values of p are the relative resistances at the different tempe- 

 ratures of one cubic centimetre of the metal taken at the 

 atmospheric temperature and allowed to shrink or expand 

 with the temperature. They are therefore the relative specific 

 resistances for constant mass and not for constant volume. 

 These latter cannot be obtained until we know the true mean 

 coefficient of cubical or linear expansion of the various metals 

 and alloys between 0° C. and -100° C. and -200° C. ; but 

 in any case, even for the most expansible metal, the correction 

 would probably not amount to one-half of a per cent., or to 

 less than the error created in the measurement of the resist- 

 ance by an uncertainty in the temperature to the extent of 

 one degree Centigrade. 



§ 4. The following Tables give these specific resistances 

 for constant mass (p) at the various temperatures (t°) of the 

 different pure metals, alloys, and impure metals employed. 

 The mean value of the various results is given, and the 

 temperature in degrees Centigrade placed over the number de- 

 noting the specific resistance in absolute electromagnetic units. 



If these specific resistances are plotted out in a series of 

 curves, taking the absolute temperature as abscissae, we find 

 that all the lines of resistance are more or less curved lines 

 which tend downwards in such a way as to show that if pro- 

 longed beyond —200° C. they would probably pass through 

 or near the origin or absolute zero. These curves of resist- 

 ance can be divided into three classes : — (i.) those of metals 

 such as iron, nickel, tin, and perhaps copper, which are con- 

 cave upwards ; (ii.) those of metals such as gold, platinum, and 

 palladium, and probably silver, which are concave downwards 

 towards the axis of temperature ; and (iii.) those of metals 

 such as aluminium, which are apparently nearly straight lines. 



In the case of a metal of the first class, such as iron, the 

 resistance changes with the temperature in such a way that 

 the rate of change of resistance with temperature increases as 

 the temperature increases. In other words, the second dif- 

 ferential of resistance with respect to temperature is positive. 

 In the case of a metal of the second class, such as platinum, 

 the second differential of resistance with temperature is nega- 

 tive ; that is, as the temperature increases the rate of change 

 of resistance with temperature decreases. 



This distinction between such metals as platinum and nickel, 

 in respect of their variation of resistance with temperature, 

 has been noted by Professor Cargill G. Knott (Proc. Roy. 

 Soc. Edin. vol. xxxiii. 1888, p. 187), in a memoir on the 

 Electrical Eesistance of Nickel at High Temperatures. Prof. 

 Knott's observations on the resistance of nickel were made 

 between 0° and 300° C. Our own were between + 100° and 



