Producible by Attrition and Contact of Metals. 23 



aud then the needle will stand at zero. In this case heat from the atmosphere 

 enters the portions of metal not covered by water; it is then communicated to, 

 and absorbed by the water ; thus, a retrograde current of heat is created, and 

 hence the opposite deflection. The transmission of heat by the metals is un- 

 equal on account of their unequal conducting powers and capacities. 



It is a consequence of this law that, in general, if two metals in contact, con- 

 nected with tlie galvanometer by capillary rheophores, be equally subjected to 

 the action of the same degree of heat or cold throughout their whole mass, as 

 by being immersed in hot or cold water, there will be, at first, a deflection, 

 which, however, will speedily cease, and the needle will settle at zero ; heat was 

 either entering or leaving the metals unequally, hence the opposite deflections 

 according to the current of the heat ; but all deflection will cease when the ca- 

 pacities for heat are satisfied. 



These transitory deflections have misled some philosophers into the belief that 

 they are produced by an equal elevation or depression of the temperature of 

 both metals above or below that of the surrounding media. If I am correct in the 

 laws laid down, and I know of no reason to doubt it, a change of temperature, 

 equal in both metals, does not impart to them the power of producing deflections ; 

 although deflections will occur when the metals are in progress of arriving at 

 equality of temperature, but, that point once attained, all deflection ceases. 



Law X. The facts on which this law is founded are very perplexing, so 

 diflicult is it to disentangle the complications which take place. A few of the 

 leading ones only shall be noticed. 



Eods of metals, imder certain circumstances, act diflerently from hemi- 

 spheres : the latter produce but one kind of deflection for each mass, at a given 

 temperature and at the same end of the coil ; the former give an opposite de- 

 flection for each end of the rod dependent on its temperature. Let a rod of 

 German silver, eight inches long, and a quarter of an inch in diameter, be con- 

 nected by a capillary copper wire to the zinc side of the galvanometer, and a 

 similar rod of bright iron to the silver side. If one end of the German silver 

 rod, adequately heated by a spirit lamp, be applied or rubbed to the cold iron 

 rod, a strong western deflection will result ; its cold end would give eastern 

 deflection. The contact being continued, let the cold end of the German sil- 

 ver rod be slowly heated, the deflection will gradually lessen, and at length 



