Dr. Draper on the Electro-motive Potsoer 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 : 25i 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 : 

 4)90 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 1 76 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- 

 pei'iment, care was taken that no heat should be transmitted 

 along the wire to the connecting cups. 



