202 Mr. J. Parker on the Theory of Magnetism and 



five quantity of work expended in it, which is in accordance 

 with the principles of thermodynamics. 



IV. Next, let both bodies be positively electrified, and suppose 

 that they are at such a distance from one another throughout 

 the experiment that the force between them is always re- 

 pulsive. Then, when B is moved from X to Y in such a way 

 that the distributions are constantly in stable equilibrium, the 

 repulsion on B in the position Y will be less than if the dis- 

 tributions had not changed. (In the case of a supposed 

 diamagnetic " soft substance, " it will be recollected that the 

 repulsive force in the position Y was greater than if there 

 had been no change in the magnetization.) From this it is 

 easily seen that electric repulsion is in accordance with the 

 principles of thermodynamics. 



V. Again, if we assume the principle of the conservation of 

 electricity, which can be partially obtained as a theoretical 

 deduction from the principle of energy and Carnot's principle, 

 our two assumptions enable us to find expressions for the 

 energy and entropy of an electrified system at rest in which 

 the distribution of electricity is in equilibrium, stable or 

 unstable. Then we can easily find the condition of electric 

 stability on an insulated homogeneous conductor. For if P,Q 

 be two points of such a conductor, our expressions show that 

 if the potential have a higher value at P than at Q, it will be 

 possible for electricity to pass, of itself, from P to Q, but not 

 from Q to P. Conversely, if the potential at P be less than 

 at Q, electricity will be able to pass, of itself, from Q to P, 

 but not from P to Q. Suppose, for example, that the potential 

 at P is greater than at Q: then a slight shake may cause a 

 passage of electricity from P to Q, and this will diminish the 

 potential at P and increase it at Q. Thus the potential tends 

 to become uniform throughout the conductor, and when it has 

 become uniform, the distribution is stable. 



VI. Again, our expressions for the energy and entropy enable 

 us to discuss the conditions of stability of two conductors A, 

 B, separately homogeneous, which are in contact with each 

 other, and isolated from all other bodies. For if P be a 

 point in the body A, and Q a point in the body B, our expres- 

 sions show that if the potential at P exceed the potential at 

 Q by more than a certain amount, electricity will be able, of 

 itself, to pass from P to Q, but not from Q to P; and con- 

 versely. Thus the condition of stability is that the potential 

 should be uniform throughout each conductor, and that the 

 two values should differ according to a certain law; all which 

 is verified by experiment. Again, if the distributions be 

 indefinitely near stable, and we suppose a small charge to 

 pass from one conductor to the other, our theory indicates 



