1903.] Thermal Relations of Energy of Magnetisation. 



239 



the coil must be infinitely small; hence t a x + 1/ll, so that — = J~ 



OX t 



when x is negligible. 



The energy of magnetisation of the core, which is JJ§US, where 



| = k1//x, and therefore is i* / ^ 7rnL \ j s no ^ included in this con- 



" [x\x + l/LL/ 



servation. Its increase for a change 8x is - - — \ S&e, which is 



4tt \ 1 1 ix ) 



large compared with the quantities above. The fraction of it, 

 which is comparable with the other variations, is compensated thermally, 

 by absorption of the heat of the system, and has, therefore, only the 

 limited availability of thermal energy. The remainder belongs 

 intrinsically to the magnetisation, constituting mutual energy of con- 

 tiguous molecules ; how much of it, as above expressed, is of thermal 

 origin remains undetermined in the absence of calorimetric experiment. 



8. The main points that it has been sought to bring out are as 

 follows : — 



(i) In an electrodynamic field there exists the usual specification of 

 electrokinetic energy, but also in addition the energy of magnetisation 

 of magnetic material. 



(ii) This energy of magnetisation appears as made up of a part given 

 by the ordinary formula, which (when paramagnetic) is derived from 

 thermal sources, and so in the absence of hysteresis has the limited 

 mechanical availability of thermal energy ; together with a local part 

 which is to some extent thus available, but is also in part permanent 

 intrinsic energy of the molecules, regarded temporarily as magnetic 

 energy. 



(iii) The law of Curie, that the susceptibility of weak paramagnetic 

 substances is inversely proportional to the absolute temperature, is 

 involved in these statements. 



(iv) The extent of the direct (non-thermal) availability of retained 

 magnetism can be inferred only by empirical procedure, for example, 

 in general features by inspection of the hysteresis diagram, as pointed 

 out by Lord Rayleigh. 



