±\ 



towards the Theory of Thermo-electricity. 207 



instantaneous movement of the needle as evident in one case 

 as in the other. So far, then, as promptitude of transmission 

 through long wires is a distinction, thermo-electricity does not 

 differ from the other kinds. 



The magnetest employed in these experiments consists of a 

 pair of sewing needles, 3 inches long, the lower one having 

 just enough of predominance to give them terrestrial direction. 

 The conductor, which passes only once between them, and 

 returns beneath the lower needle, consists of four lengths of 

 bright copper bell- wire laid together, not twisted, the ends 

 amalgamated, and working into mercury boxes. A pair, of 

 copper and tinned iron, by the heat of the fingers gives a de- 

 flection about 25°. 



A guide-cross lies between the mercury boxes, to keep 

 always before the eyes the relations between the current and 

 the needle. 



It is constructed of two slips of 

 card, fastened in the centre, one 

 over the other, with sealing-wax; 

 one representing the magnet, the 

 other the conducting wire. On 

 the latter an arrow is drawn (on 

 both sides) to indicate the direc- 

 tion of the current; on the former 

 a line, also on both sides, to di- 

 stinguish the marked pole. 



This cross turned over, on either of the slips, as an axis, 

 or placed horizontally, vertically, or in any degree of obliquity, 

 always exhibits the relation between the current and the needle: 

 and being laid down, with its magnet corresponding with the 

 position of the needle, it shows in what direction the current 

 is passing over it; or turning it over upon the magnet as an 

 axis, the reverse current is seen returning under it. 



2. If there be any modification of electricity answering to 

 that of Dr. Hare, a compound of electricity and caloric, this, 

 from the mode of its generation, should be the one. But the 

 same pair, of antimony and bismuth (1.), heated until they would 

 drive the needle nearly round the circle, by the first impres- 

 sion, had not any appreciable effect on Mr. Harris's thermotest, 

 though tried upon wires of various dimensions and different con- 

 ducting powers. Thermo-electricity, then, seems to be no way 

 distinguished as containing caloric. 



Attractions and repulsions, or chemical decompositions 

 would be precluded by its low tension ; and these are its only 

 apparent distinctions from the voltaic current, or from the 

 electricity of the machine. 



