Producible by Attrition and Contact of Metcds. 27 



ends of the compound piece were connected with the galvanometer by means 

 of capillary copper wire. The point of junction of the V was then immersed 

 in boiling water to the depth of an inch; there was an immediate deflection to 

 the amount of 90° west. The compound piece was then totally immersed in 

 the water; the needle came round to 15^ east.* lu these cases the rhcophores 

 were proved not to have had any influence; and chemical action of the water 

 was out of the question, as the metals were in strict contact. 



In the enunciation of the Law, it is stated that this deflection, produced by 

 total immersion of the associated metals, will be of temporary duration. It 

 might appear that the occurrence of any deflection, whether the rods were par- 

 tially or totally heated, would be a contradiction of Law iv., for it might be 

 supposed that as the two metals were exposed to the same temperature, their 

 contact should not cause deflection. But although both metals were exposed 

 to the same source of heat, it is a well-known fact that they do not each absorb 

 lieat in such a manner as to produce equal temperatures in equal times ; the 

 actual temperature of each metal, owmg to this different influx of heat, is there- 

 fore unequal ; more or less deflection must consequently ensue ; and will con- 

 tinue up to the moment when the capacities for heat of the two metals are satis- 

 fied, which will be in a longer or shorter time, according to circumstances. 

 The metals having then arrived at equality of temperature, the needle will stand 

 at zero. Thus, it is plain that between the present Law and the fourth Law 

 there is no discordance ; all the remaining parts of the former are intelligible 

 without further explanation. 



Laws XII., XIII., XIV. need no comment. 



Laws XV. and XVI. The consideration of these Laws involves the great 

 question of all, — Are the deflections produced by thermo-contact and attrition 



• With some associations of metals it is exceedingly difficult to obtain these results. A rod 

 of tin and one of lead require a galvanometer of exquisite susceptibility. Eods of German silver 

 and iron are apt to deceive; but by dipping the tied ends into boiling water, taking care to keep 

 the remainders cold, and after marking the deflection, removing the tied rods, and leaving them to 

 cool perfectly for some hours, and then plunging them, along with the rheophores, suddenly and 

 totally into boiling water, the opposite deflection will be obtained. This indeed is the best method 

 for all metals. 



E 2 



