458 Dr. Draper on the Electro-motive Povoer 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 very yoint 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. 
