of Electro-Magnetism by Heat. 49 
with silver, platina, and palladium, as the wires were silver or 
platina. In these instances it seems remarkable, not only, that 
the bar appeared to change its electrical states with different 
wires, but that electricity or rather electro-magnetism should be 
exhibited when the bar and wire were of the same metal. In 
the first case, it might be supposed that the electricity was excited 
by the contact of dissimilar* metals; as, in the galvanic circuit, 
copper is positive to zinc, but negative to silver. But this hypo- 
thesis is inapplicable to cases of the second description. If the 
effect depended on the contact of dissimilar metals, it would be 
greatest between those which are opposed in the galvanic circuit, 
and would cease when the bar and wire were of the same sub- 
stance. On making the trial with a bar of zinc and silver wires, 
the deviation was not greater than that by the same bar and wires 
of zinc. Again, platina and silver are both positive with reference 
to copper, yet the deviations were opposite; and silver and copper 
bars acted strongly with silver and copper wires respectively. As 
in all these instances, to prevent ambiguity, the wires were not 
soldered, but rivetted to the bars, I cannot but conclude, that 
the hypothesis of electricity being excited by the contact of dissi- 
milar metals, is, whatever plausibility it may possess in other 
circumstances, inapplicable to the case of its developement by 
heat. The following experiment is, I think, decisive. — Two 
wires, each composed, the one half of platina, the other of silver, 
soldered together in the middle, were rivetted into a bar of 
brass. When the silver ends were connected with the brass, the 
deviation was positive; on reversing each wire, and therefore con- 
necting the platina and brass, it was negative; still retaining the 
platina contact, but shortening the platina wire to about half an 
inch, the deviation again became positive. In every case the 
brass was in contact with a metal highly positive with respect 
* Dissimilar as to their galvanic relations. 
Vol. Uf. Part I. G 
