the Intrinsic Field of a Magnet. 417 



The critical temperature of* cobalt is about 1100° C, and at 

 this high temperature Pionchon remarks that investigations 

 on specific heats become very difficult and the results must 

 be accepted with some reserve ; there are no observations 

 for cobalt in sufficient detail to allow the sudden fall of 

 specific heat above the critical temperature to be traced 

 exactly. 



Magnetite gives precisely the required ratio of 3 to 5 for 

 the specific heats at 17° C. and at its critical temperature, 

 but nickel and cobalt have not so exactly this ratio. If, 

 however, the specific heats of nickel and cobalt are com- 

 pared at corresponding temperatures, the ratios of their 

 specific heats are more nearly alike, and if the chosen 

 corresponding temperature at the lower point is that of 

 iron at 17° 0., they are more nearly 3 to 5. 



Iron, which has nearly the saine specific heat as nickel at 

 17° C, has an abnormally high specific heat at the critical 

 temperature, which is very nearly the double of that of 

 nickel, and the ratio of the specific heats, namely 3 to 9' 3, 

 is thus double the ratio of 3 to 4*7 for nickel. This may be 

 explained by assuming, as before, that the molecule of iron 

 is subdivided at the critical temperature and, if into two parts, 

 the number of degrees of freedom would be doubled and the 

 specific heat would be doubled. 



There are two interesting facts which the table brings 

 out, which can be no more than mentioned here: first, the 

 rise or the fall of the specific heat is in each case almost an 

 exact multiple of 0*027, the smallest number ; and, secondly, 

 the abrupt fall above the critical temperature is always the 

 half of the rise. 



9. Electrical Resistivity. — If resistivity is expressed in terms 

 of the heat which is emitted from a conductor, it is evident 

 that a change in the specific heat must produce a change in 

 the resistivity. If s is the specific heat, p the resistivity, 

 and t the temperature, then. the relation may be put, 



— z ^ 



where k is a constant. 



As long as s is constant, the resistivity will vary linearly 

 with the temperature, but if s increases, the slope of the line, 

 giving the relation of p to T, will also increase as the 

 diagram shows (fig. 3). Now, the specific heat of nickel 

 changes, as we have seen, from air temperature to the 



Phil. Mag. S. 6. Vol. 43. No. 255. March 1922. 2 E 



