458 Dr. Draper on the Electro-motive Power of Heat. 



The same was repeated with a piece of iron wire, of the 

 same length and under the same circumstances. The cur- 

 rent 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. 



To the increased resistance to conduction, occasioned by 

 an increased temperature, we are to impute the slight rise of 

 tension observed in thermo-electric currents. The quantities 

 are of the same order. 



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 effica- 

 cious as an indefinitely extended surface." 



The quantities of electricity evolved by hydroelectric pairs 

 has been shown to increase with their surfaces, but it is not 

 so in thermo-electric arrangements. A pair of disks of cop- 

 per and iron, two inches in diameter, were soldered together ; 

 they had continuous straps projecting from them, which served 

 to connect them with the galvanometer 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. 



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° Fahr., I dip- 

 ped into it a second copper wire, the temperature of which was 

 about 60° Fahr. ; the galvanometer needles moved through 

 15 degrees. 



2nd. 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. 



3rd. But when I touched the surface of the mercury with 

 the xjery 'point of the cold wire, there was a deflection of 60 

 degrees. 



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. 



