96 QUANTITY OF ELECTRICITY INDEPENDENT OF HEATED SURFACE. 



361. The increase of tension here detected depends unquestionably on increased re- 

 sistance to conduction, which the wires exhibit as their temperature rises, as the follow- 

 ing experiments show. 



362. A pair of copper and iron evolved a current at the boiling point of water, which, 

 passing through a wire of copper eight feet long, was determined at the galvanometer to 

 be 176 degrees. Having twisted a part of this wire into a spiral, so as to go over the 

 flame of a spirit-lamp, 8 inches of it were thereby brought to a red heat ; the deviation 

 of the needle fell now to 165, being a deficit of 11 degrees. In thi,s experiment, care was 

 taken that no heat should be transmitted along the wire to the connecting cups. 



363. The same was repeated with a piece of iron wire, of the same length and un- 

 der the same circumstances. The current at first being 90 degrees, as soon as the spiral 

 was made red hot, it fell to 61 degrees, being a deficit, therefore, of nearly one third the 

 whole amount. 



364. To the increased resistance to conduction, occasioned by an increased tempera- 

 ture, we are to impute the slight rise of tension observed in thermo-electric currents. 

 The quantities are of the same order. 



365. We have next to show " that the quantity of electricity evolved at any given 

 temperature is independent of the amount of heated surface ; a mere point being just 

 as efficacious as an indefinitely extended surface." 



366. The quantities of electricity evolved by hydro-electric pairs has been shown to 

 increase with their surfaces, but it is not so in thermo-electric arrangements. A pair 

 of disks of copper and iron two inches in diameter were soldered together ; they had 

 continuous straps projecting from them, which served to connect them with the galva- 

 nometer cups. At the boiling point of water they gave 62 degrees ; on being cut down 

 to half an inch in diameter, they still gave 62. On the disk being entirely removed, 

 and the copper made to touch the iron by a mere point, its extremity being roughly 

 sharpened, the deflection was still 62. 



367. By means of a common deflecting multiplier, I obtained the following results : 

 1st. A copper wire being placed in a bath of mercury, the temperature of which was 

 240 Fah., I dipped into it a second copper wire, the temperature of which was about 

 60 Fah. ; the galvanometer needles moved through 15 degrees. 



2d. The cold wire being sharpened to a point, and plunged deliberately into the 

 mercury to the bottom of the bath, the deflection was 19 degrees. 



3d. But when I touched the surface of the mercury with the very point of the cold 

 wire, there was a deflection of 60 degrees. 



368. Having laid a plate of tinned iron upon the surface of some hot mercury, it 

 was touched with the point of the cold wire. There was a strong deflection of the 

 needles in the opposite direction to what would have been the case had the mercury 

 been touched, and not the iron. The under surface of the iron was therefore acting as 

 a hot face, and the parts round the point as a cold face, being temporarily chilled by the 

 touch of the wire. 



369. These results explain the anomalies observed by some of those who investi- 

 gated the course of thermo-electric currents by means of small metallic fragments. 



