442 Prof. E. Edlund on the least Electromotive Force 



first part of the resistance in question be called a, and the other 

 part, corresponding to a luminous arc of unit length, is b 3 the re- 

 sistance of a luminous arc whose length amounts to n units is 

 a-\-nb. It follows from the experimental part of the research in 

 question that a is inversely as the intensity. It is therefore 

 easy to calculate the length of a for a certain intensity, provided 

 its value is known for another. The resistance b also increases 

 in magnitude if the intensity decreases, although the law for this 

 increase could not be deduced from the experiments, because b 

 for the unit of length assumed was so small, in comparison with 

 a, that no accurate measurements in this respect could be made. 



If, then, the length of the luminous arc be increased while at 

 the same time the intensity is not kept constant by shunting 

 out some resistance, the intensity is diminished for two reasons : 

 first, the quantity a-Vnb is increased by n being increased, 

 even if a and b were unchangeable; and, secondly, a and b in- 

 crease in magnitude if the intensity is diminished. 



The circumstance alone that a is independent of the length of 

 the luminous arc throws suspicion on the nature of its resistance. 

 In this manner a voltaic resistance would have been found which, 

 independently of other resistances, would be independent of the 

 length of the conduction. In the research in question I have 

 shown how, by the aid of the mechanical theory of heat, it may 

 be demonstrated that the diminution in the intensity of the cur- 

 rent represented by a is not caused by a resistance, but is due 

 to an electromotive force which sends a current in an opposite 

 direction to the principal current. If, now, the magnitude of 

 the electromotive force be calculated which would produce as 

 great a diminution in the intensity as the resistance a (which is 

 inversely as the intensity), we find that this electromotive force 

 (D) is independent of the intensity. But this conclusion, that D 

 is independent of the intensity, or, in other words, that a is 

 inversely proportional to it, depends on experiments which were 

 made with powerful currents ; practical difficulties presented 

 themselves in the attempt to keep the luminous arc constant for 

 some time when the intensity was small; hence it might be 

 very possible that D was only constant at a certain limit of in- 

 tensity, but varied with the intensity below this limit. This 

 seemed to me not at all improbable, especially as some similar 

 phenomena exhibit a similar deportment; the force of voltaic 

 polarization, for instance, increases with the intensity up to a 

 certain limit, and is then constant. Hence I determined to 

 make new observations. 



2. Since the method which I used in my former research to 

 determine D is inapplicable (for the reasons adduced) when the 

 intensity is small, another method had to be employed. If D is ac- 



