Page:Text-book of Electrochemistry.djvu/221

 2o6 ELECTROMOTIVE FORCE. chap.

At that time it was believed, as Yolta had assumed, that the seat of the electromotive force was at the place of contact of the two metals, in the Daniell cell at the place of con- tact of the copper and the ziuc; the electromotive force between these two metals was supposed to be about 1 volt. Edlund (4) determined how much heat is evolved when a definite quantity of electricity is passed through a junction of copper and zinc (the Peltier effect). According to the above principle we should be able to calculate this quantity of heat. The observed result was, however, very much smaller than expected, the potential difference for Cu | Zn being only 0006 volt instead of 1 volt. This result induced Edlund to investigate the correctness of the Thomson rule.

In his discussion he made use of some results which had been obtained by Eaoult. Eaoult observed that in the electrolysis of water in a voltameter, besides the evolution of gases, there is a local heating which is not due to the friction of the ions against the liquid, and is therefore different from the Joule effect. In these experiments Baoult also measured the electromotive force.

Edlund gave the following explanation. In the volta- coulombs are moved against this force, then for each volt there will be an evolution of heat amounting to 23,070 cal. (23,900 cal. for 1 dll., according to the numbers then accepted). In one case, for instance, the back electromotive force was 204 dll., and there should therefore be an evolu- tion of 48,756 cal. If we subtract from this the quantity of heat, 34,462 cal., required for the decomposition of the water, we obtain for the local heating 14,294 cal., whilst Baoult found 14,898 cal. Several similar experimental results of Eaoult were calculated with the same success (1869). Edlund later showed that in the electrolysis of silver salts a local cooling may take place. Xow, if Thomson's rule were correct, such local heat effects should not occur, but the heat necessary for the decomposition should be exactly sufficient to produce the electromotive force of the element.

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