Dr. Draper on the Electro-motive Povoer of Heat, 457 
1st. At -the boiling point of water, a pair of copper and 
platina, the unexcited extremity of which was carefully main- 
tained at 67° Fahr., evolved as a mean of four trials, three 
of which were absolutely identical, 123 degrees of electricity, 
of which 23 could pass a secondary wire. 
Then, by the aid of the furnace and sand-bath, the tem- 
perature was raised until the pair evolved 783 degrees, as a 
mean of four trials ; of these 163 could pass the secondary wire. 
Now, 
As 783 : 163 : : 123 : 25|- instead of 23 
showing therefore a slight rise of tension. 
2nd. The pair of copper and iron gave at the boiling point 
of water 300 degrees, of which 57 passed the secondary wire. 
The temperature was now raised, with the following results : 
490 degrees passing the primary, 95 the secondary wire. 
553 — 
— 
— 
113 
545 — 
__ 
— 
112 
493 — 
— 
— 
110 
It will be understood, that although the quantities of elec- 
tricity indicated in the first column do not regularly increase, 
that the temperatures were notwithstanding going regularly 
upwards : to this peculiarity of the systems into which iron 
enters I have already alluded. Let us now compare these 
measures with those obtained for the boiling point of water : 
As 490 : 95 : : 300 : 58 instead of 57. 
553 : 113 : : 300 : 61 — 
545 : 112 : : 300 : 61 — 
493 : 110 : : 300 : 67 — 
we find, therefore, that in the case of both these systems of 
metals, the tension slowly rises with increase of temperature, 
being much better marked in the latter than in the former 
instance. 
The increase of tension here detected, depends unquestion- 
ably on increased resistance to conduction, which the wires 
exhibit as their temperature rises, as the following experi- 
ments show. 
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 de- 
grees. 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 this ex- 
periment, care was taken that no heat should be transmitted 
along the wire to the connecting cups. 
