of Electro-Magnetism by Heat. 51 
connected at their heated extremities; or, which is the same thing, 
if a single bar be heated at the middle, no effect is produced, the 
equal and opposite currents counteracting each other: if under 
the same circumstances, the metals are dissimilar, the effect is 
that arising from the joint action of their conspiring currents. 
In some respects this arrangement is analogous to that of the 
galvanic circuit; heat in one case acting the part of an acid in 
the other; but there is one material difference between them. 
In the first, the metallic circuit is complete, and the current is, 
as has been already observed, in the same direction throughout 
every part of it. In the second, the circuit is interrupted, and 
the current through the acid is opposite to that through the wire.* 
(Fig. 4 and 5). 
If the bar of antimony AB above mentioned be broken un- 
equally into two parts ab, cd, (Fig. 6.), and these be connected 
by a copper wire; on heating one part and cooling the other re- 
gularly throughout, no effect is produced, however short the interval 
may be between them.t If the parts ab, cd be (Fig. 6.) again 
* This may be readily shewn by connecting S and Z in Fig. 5. by a fine wire. 
If two wires, the one platina, the other bismuth, be connected with the galvanoscope, on 
immersing their other extremities (not in contact) for a short time in nitric acid, electro- 
magnetic effects are produced by their galvanic action; on making the contact the effect 
still continues, and in the same direction, but arises from heat; if platina and iron be 
used the second action is contrary to the first. 
+ As electricity is excited by the contact and separation of two polished disks of zine and 
copper, Mr. Herschel suggested to me, that it might be desirable to try what effect would be 
produced, by heating one of the disks previously to its application to the other. If their 
thickness be inconsiderable in comparison with their surface, the electro-magnetism elicited 
is searcely, if at all perceptible; if they be of considerable thickness, and both bismuth 
or both antimony, it is greatly increased; and again is materially diminished if one be 
antimony and the other bismuth. he reason is obvious: in the first case each disk receives 
almost instantaneously an uniform temperature throughout; the second case becomes that 
of two bars of the same metal, having their extremities at unequal temperatures in contact, 
and their electric currents in the same direction; in the third case they are opposed. All 
these instances are decidedly unfavourable to the supposition of electro-magnetism being evolved 
by the contact of dissimilar metals, 
G2 
