268 Mr. Prideaux's Experime?ital Contributions 



the currents was traced by the needles last described (26.). 

 The ends of the copper wires were now brought into the line 

 of the current, (by turning and sliding the hoops,) in the hope 

 that, by the superior conducting power of the copper, either 

 the direct or return current might be taken up by the wire. 

 But I could not succeed in obtaining this, or the vestige of any 

 current, through the wire, until, coming in contact with a 

 warm part of the bismuth, it assumed its proper thermo- 

 electric action. 



28. Two square bars, each connected at foot with the mag- 

 netest, and the head of one heated, were brought into contact 

 at the upper ends : the current always set from the hot into the 

 cold bar (21.), though in the majority of cases it would have gone 

 down the heated face of the hot bar, and up the cold face. 



29. On heating one of these bars at midlength, so that the 

 heat should not readily reach either end, the current was al- 

 ways from the cold to the heated bar, although in that single 

 bar (/. e. in either singly,) it would have been in different di- 

 rections, according to the side heated. 



30. Two bismuth rods, each 8xj inches, were tied together 

 at top, the feet communicating with the magnetest: on direct- 

 ing the blow-pipe dart to the midlength of either, a current 

 immediately set from the cold rod to the one so heated, as in 

 (29.) ; but as the heat extended itself through the rod, the 

 needle gradually returned to its station, and passed to the other 

 side, as usual with two bars of that metal, one hot, the other 

 cold, at the point of contact (21. 28.). I obtained no satis- 

 factory evidence of any current analogous to (29. 30.), by 

 heating the midlength of a pair of any uncrystallized metal. 



31. No evidence, then, appears of any connexion between 

 the currents induced in a mass of bismuth, and those pervading 

 the same metal, when forming part of a circuit. The former 

 seem related to the crystalline laminae, the latter independent 

 of the laminar direction, and generally analogous to those in 

 other metals, the structure of which is not crystalline. 



The changes which take place in zinc may probably offer 

 fairer chance of success (22.); and it is for the purpose of in- 

 viting the attention of experimenters to them, that I offer this 

 inquiry to the public in its present stage. 



VII. Of the effects of extraneous contact on a thermo-electric 



circuit. 



32. It is quoted above from Becquerel (13.), that a con- 

 tinuous circuit of one metal, heated, and touched near the 

 heated point with a cold piece of the same, gives a current 

 from the hot point to the part in contact with the cold piece. 



