474 



Concerning the molecular lield we will assume that it consists of 

 couples due to mutual actions of the molecules which are not magnetic 

 actions, and that on the assumption B it is out of the question that the 

 molecular field could change the moment of a magnet by induction. 

 The energy per unit of volume due to the molecular field may be 

 written as 



— i kM\ 



if kM represents the molecular field. 



We now follow the reasoning of § 52a. Hence we considei- a 

 physically infinitely small spher3 of volume B within the magnetized 

 body. The field within it, due to what lies outside it, is indicated 

 by 2, the field due to what lies inside the sphere by 1. 



The energy within the sphere due to 2 may be called -. Ba, 



that ,. ,, ,, ,, ,, the superposition of 2 and 1:^/^, 



1 • Be 



In calculating the latter we distinguish : 



the own energy of each elementary magnet contained in B indivi- 

 dually ; the sum of these may be called Bp; 



the energy due to the superposition of the fields of the different 

 elementary magnets: Bq. 



We calculate p and q for the whole space (outside B only ether 

 being assumed) and then have to subtract from this sum the energy 

 in the outside space furnished by the elementary magnets that are 

 inside the sphere. Let this be Br. 



Then 



c=zp -\- q — r. 



Let further BU,. be the energy of the (rotatory) heat motion, 

 which opposes the. directing of the particles {U,- is reckoned here 

 per unit of volume). 



Then the whole energy per unit of volume is: 



U = a^ h + p -^ q — r -- \ kW ^- U,-. 



By the own energy p is to be understood that which is due 

 to the constitution of the elementary magnet and to its own field. 

 As on the assumption A nothing ever changes of the elementary 

 magnet, p is an absolute constant which need not be further considered. 



On the assumption B, by p is understood the energy (magnetic 

 and eventually ordinary kinetic) which corresponds to the rotation 

 round the axis a (^ 48é?). This energy is supposed not to be included 

 in Ur- As the velocity of that rotation changes by induction, p is 

 not an absolute constant on this assumption. 



