Producible by Attrition and Contact of Metals. 7 



Law VIII. Whether these deflections will take place on the eastern or 

 western side of the magnetic meridian will be determined by the relative tem- 

 peratures at which contact or attrition of the metals lias been effected ; and by 

 the peculiar influence of the metal that is placed in connexion with each 

 extremity of the coil of the galvanometer. 



Law IX. The condition necessary to the production of deflection, by con- 

 tact of two different metals, is tliat heat sliall be at that moment entering or 

 leaving one of them ; or that heat shall be unequally entering or unequally 

 leaving both of them ; no matter whether the inequality depend on difference 

 of supply, of conduction, or of capacity; or on unequal diffusion of heat arising 

 from difference of mass of the metals ; or on more than one, or all of these 

 causes conjointly. The deflection, caused by the unequal entrance of heat into 

 metals in contact, will be on the side of the magnetic meridian opposite to that 

 on which it would be if heat were leaving them unequally. 



Law X. The form of the metal influences its effects: rods and rings for 

 the most part act differently from hemispherical masses. If a metal rod be 

 heated at one end, and applied or rubbed to a rod of a different metal at a 

 much lower temperature, the former will most generally be thrown into a state 

 of polarity, each end being capable of producing deflection opposite to that 

 which the other end produces: the latter rod when similarly treated will evince 

 a contrary polarity. Either rod will reverse the deflections mentioned if its 

 other end be the sole heated one. But the application of an equal and ade- 

 quate heat to the whole extent of a rod will cause it, in the case of some metals, 

 not only to lose its polarity, but even the power of producing deflection, the 

 needle remaining at zero ; and in other cases, although the polarity will be 

 destroyed, the rod will act in the same manner and with the same energy, with 

 regard to producing deflection, as a hemispherical mass would have done at 

 the same temperature. In many cases the polarity may be destroyed, and all 

 efiect of that metal rendered null, by the nice adjustment of the relative por- 

 tions of the rod heated and not heated. 



Law XL If two rods of diSerent metals, properly connected with the gal- 

 vanometer, be placed in contact with each other at one point ; and if a corre- 

 sponding small portion of each be subjected, at the point of contact, to an equal 

 and adequate temperature, above or below that of their respective remainders, 

 they will produce deflection on the side of the magnetic meridian opposite to 



