of Thermo-electric Phenomena. 537 



up of two parts, such that r—r f -\-r ff , whereof/ is the specific 

 resistance referred to positive electricity, and r" is the specific 

 resistance referred to negative electricity, and that in any 

 metal in which a current exhibits convective power for heat 

 the two parts are not equal. If the " specific heat of vitreous 

 electricity " is positive (as Thomson has experimentally proved 

 it to be in copper), r" is greater than /; but if it is negative 

 (as it is in iron), then r 1 is greater than r" . However, this 

 will appear more fully presently, § 35. 



§ 31. Consider now the electrical condition of a homo- 

 geneous mass of metal. Its surface molecules are moving 

 more quickly outwards than inwards, and are hence straining 

 the cords outwards (which cord they strain most depends upon 

 circumstances ; but for simplicity I will suppose that there is 

 only one, and that the positive cord) : the cord inside is thus 

 in a slight state of tension, or (§ 1) the potential of the metal 

 is lower than that of the air in its neighbourhood. The cord 

 does not move, because the surface forces are balanced all 

 round : it resembles a drop of water in a capillary tube. (If 

 the negative cord were acted on most, the condition of the 

 metal would resemble mercury in a capillary tube, or simply 

 a drop of water in its own skin.) Another mass of different 

 material will show the same thing, only to a different amount. 

 If the two metals are put into contact, we have three kinds of 

 bounding surfaces, viz. those which separate air and A, A and 

 B, B and air ; and the forces at each surface are in general dif- 

 ferent : hence the forces all round A are no longer balanced, and 

 the cord moves, say from A to B, a certain limited extent, just as 

 the water in two capillary tubes of different bore would when 

 they are brought into contact. The particles of the dielectric 

 outside (being also threaded on the cord) will at the same 

 time be pulled nearer to A and pushed further from B ; that 

 is, §§ 1 and 2, the surface of A is charged negatively and 

 that of B positively. 



§ 32. The difference of potential produced by the above 

 surface-action is closely allied to Volta's contact electricity, 

 which is also an effect taking place at the free surfaces of the 

 two metals, bat which is probacy greatly assisted by an in- 

 cipient voltaic current taking place through the air which acts 

 as the exciting fluid, the electromotive force being produced by 

 the tendency of the air to unequal chemical action on the two 

 metals. Since the air is a dielectric and not an electrolyte, 

 there is no continuous current developed, but only a difference 

 of potential capable of causing a current if the resistance of 

 the air were diminished sufficiently; and the cord is displaced 

 through the air and across the junction of the two metals to 



