Homogeneous Bodies ; with illustrative Experimenis. IS 



bismuth becomes magnetic by heating the smallest possible 

 point. 



The phgenomena generally, are sufficiently striking to be 

 exhibited in a lecture-room, and to be observed by the most 

 distant auditor, provided the needle be neutralized. Two, 

 or more rectangular frames may be selected, and properly 

 adjusted side by side, so that their combined energies may be 

 made to operate simultaneously on a very large needle. In 

 this way the experiments may be made on a very extensive 

 scale. 



Fifth Class of Experiments. 



ii. The experiments which I am now about to introduce 

 present phaenomena, perhaps somewhat less complicated, 

 though by no means less curious, than those I have already 

 described. 



I had observed, whilst experimenting with a rectangular 

 frame of bismuth of large dimensions, that the needle would 

 sometimes be deflected in one direction and sometimes in the 

 other, even when the point of heat was not varied. Struck 

 by this unusual phaenomenon, I proceeded to examine it with 

 some degree of minuteness, and with an intention of ascer- 

 taining its cause. 



45. Before noticing this apparent anomaly, I had con- 

 stantly held the plane of each rectangle in the plane of the 

 magnetic meridian, and with its lower edge as close as possi- 

 ble to the needle,— a position, which I considered as the most 

 Ukely to obtain true results, because, when so placed, the 

 upper edge of the frame, in consequence of its great distance, 

 could not affect the deflections of the needle produced by the 

 thermo-magnetic forces in the lower side. This position of 

 the frame would certainly have been better than any other for 

 ascertaining the direction of single electric currents, or those 

 thermo-magnetic forces which are circumfused in the appa- 

 ratus of Seebeck, and other similar combinations; although 

 a slight degree of inclination either to the east or west, with 

 such compound apparatus, would not very materially have af- 

 fected the results. I soon became convinced, however, that 

 the anomaly which 1 liad noticed proceeded entirely from 

 that cause; for if the plane of the rectangle inclined to the 

 east, the needle would be deflected in a contrary direction to 

 that which it assumed, by inclining the plane of the rectangle 

 to the west. By several trials it was found, that when the 

 plane of the frame became nearly horizontal eastward (still 

 keeping the end north), a greater deflection was obtained than 

 by holding it at any oilier angle on that side of the nieriiiian. 

 And by placing it in a similar position to the west of ihe me- 

 ridian, 



