238 The Mathematical Electricians of the 



By taking a large number of such pairs of magnetic molecules, 

 originally oriented in all directions, and at such distances that 

 the pairs do not sensibly influence each other, we may 

 construct a model whose behaviour under the influence of 

 an external magnetic field will closely resemble the actual 

 behaviour of ferromagnetic bodies. 



In order that the magnets in the model may come to rest 

 in their new positions after reversal, it will be necessary to 

 suppose that they experience some kind of dissipative force 

 which damps the oscillations ; to this would correspond in 

 actual magnetic substances the electric currents which would 

 be set up in the neighbouring mass when the molecular 

 magnets are suddenly reversed ; in either case, the sudden 

 reversals are attended by a transformation of magnetic energy 

 into heat. 



The transformation of energy from one form to another is a 

 subject which was first treated in a general fashion shortly 

 before the middle of the nineteenth century. It had long been 

 known that the energy of motion and the energy of position 

 of a dynamical system are convertible into each other, and 

 that the amount of their sum remains invariable when the 

 system is self-contained. This principle of conservation of 

 dynamical energy had been extended to optics by Fresnel, who 

 had assumed* that the energy brought to an interface by 

 incident light is equal to the energy carried away from the 

 interface by the reflected and refracted beams. A similar 

 conception was involved in Eoget's and Faraday's defencef of 

 the chemical theory of the voltaic cell ; they argued that the 

 work done by the current in the outer circuit must be provided 

 at the expense of the chemical energy stored in the cell, and 

 showed that the quantity of electricity sent round the circuit 

 is proportional to the quantity of chemicals consumed, while 

 its tension is proportional to the strength of the chemical 

 affinities concerned in the reaction. This theory was extended 



*Cf. p. 133. tCf. p. 203. 



