Homoge7ieous Bodies ; v)ith ilhistrative Experiments. 121 



106. The thermo-magnetic phaenomena were promptly and 

 uniformly displayed by this mass of zinc, and were precisely of 

 the same character as those which I have observed in experi- 

 ments with all similar masses that 1 have yet examined. They 

 have an evident reference to the point of heat; and I believe 

 they may be taken as a general standard for those which would 

 be developed by all similar masses when operated on by the 

 same process. 



107. The experiments were made by heating one corner of 

 this mass of zinc in a common fire, and keeping the other 

 parts of the metal as cool as possible. When thus heated, the 

 mass was held in various directions over the magnetic needle, 

 the deflections of which were taken as an indication for the 

 direction of the electric currents. In this manner the thermo- 

 magnetic powers of the zinc were ascertained, whilst it was 

 partially heated at its several angles in succession. 



108. If c, b, c, d, fig. 19. be permitted to represent one of 

 the flat faces of the zinc-plate, then the arrows in that figure 

 will give a tolerable representation of the directions of the 

 electric forces, as indicated by the deflections of the compass- 

 needle when the point of heat was at the angle d. By con- 

 templating this figure, it will be observed that the electric 

 forces are projected, as it were, from the heated angle into the 

 body or field of the mass, over which they become generally 

 diffused ; but separating and expanding in different directions, 

 they sweep the surface of the metal in recurving tides towards its 

 ed'res, by which routes they again return to the heated point. 



109. The straight arrows in fig. 19. would seem to indicate 

 that the electric forces in those parts of the metal were di- 

 rected in right lines, which is not strictly correct. Those 

 arrows are drawn to show the lines oi greatest energy, or those 

 parts of the metallic surface which, when presented to the 

 needle, produce the greatest deflections, and are the deter- 

 mined resultants of the numerous curvilinear forces which are 

 in active play during the transient disturbance of temperature 

 in the metal. 



110. It will appear evident by inspecting the figure, that 

 on the surface of the rectangular mass, there would necessarily 

 be two Tieidral lines, one on each side of the diagonal arrow, 

 which would be determined at right angles to the aggregate re- 

 curving electric forces, and may be represented by the dotted 

 lines d v,<l n. These lines, are those in which aniagnetic needle, 

 unsolicited by any other force, would arrange itself, and were 

 discovered by its uniform rejiose whilst situated close to those 

 parts of the metal. The situations of the neutral lines, how- 

 ever, are not constantly the same; for as tlic-y arc determined 



N.S. Vol. 10. No. 5G. Aug. 1831. li by 



