382 Prof. Thomson on the Dynamical Tlisory of Heat. 



Def. 3. The intrinsic electromotive force in any direction, at 

 any point in a solid, is the electromotive force that would be 

 experienced by an infinitely thin conducting arc of standard 

 metal, applied with its extremities to two points in a line with 

 this direction, in an infinitely small portion insulated all round 

 from the rest of the solid, divided by the distance between these 

 points. 



Def. 4. The electromotive force efficient at any point of a 

 solid, in any direction, is the difference of the electromotive forces 

 that would be experienced by an infinitely thin conducting arc 

 of standard metal, with its extremities applied to two points 

 infinitely near one another in this direction, divided by the 

 distance between the points, in the two cases separately of the 

 solid being left unchanged, and of an infinitely small portion of 

 it containing these points being insulated from the remainder, 



146. Principle of the superposition of thermo-electric action. 

 It may be assumed as an axiom, that each of any number of co- 

 existing systems of electric currents produces the same reversible 

 thermal effect in any locality as if it existed alone. 



§§ 147-155. On Thermo-electric Currents in Linear Conductors 

 of Crystalline Substance. 



147. The general characteristic of crystalline matter is, that 

 physical agencies, having particular directions in the space 

 through which they act, and depending on particular qualities 

 of the substance occupying that space, take place with different 

 intensities in different directions if the substance be crystalline. 

 Substances not naturally crystalline may have the crystalline 

 characteristic induced in them by the action of some directional 

 agency, such as mechanical strain or magnetization, and may be 

 said to be inductively crystalline. Or again, minute fragments 

 of non-crystalline substances may be put together so as to con- 

 stitute solids, which on a large scale possess the general charac- 

 teristic of homogeneous crystalline substances ; and such bodies 

 may be said to possess the crystalline characteristic by sti'ucture, 

 or to be structurally crystalline. 



148. As regards thermo-electric currents, the characteristic of 

 crystalline substance must be, that bars cut from it in different 

 directions would, when treated thermo-electrically as linear con- 

 ductors, be found in different positions in the thermo-electric 

 series; or that two bars cut from different directions in the sub- 

 stance would be thermo-electrically related to one another like 

 different metals. This property has been experimentally demon- 

 strated by Svanberg for crystals of bismuth and antimony ; and 

 there can be no doubt but that other natural metallic crystals 

 will be found to possess it. I have myself observed, that the 



