441 Prof. E. Ecllimd on the least Electromotive Force 



The experiments were made as follows*: — After the electromo- 

 tive force of the battery and the resistance had been determined 

 when the poles were screwed against each other, the electromo- 

 tive force D in the luminous arc was measured by the rules given 

 in my former investigation. For this purpose the points were 

 fitted in a pole-holder which was placed in a Foucault's lantern, 

 and the length of the luminous arc determined by measuring 

 the image projected on the screen. The pole-holder was then 

 removed from the lantern and firmly screwed against the table, 

 upon which the electromotive force of the battery was succes- 

 sively diminished by the removal of a greater or smaller number 

 of elements. At each diminution of the electromotive force, it 

 was examined whether a luminous arc could be formed or not, 

 until the limit was reached. The trough which I mentioned in 

 my former paper was used as a rheostat, and was filled with 

 sulphate of copper. In all the following experiments the line 

 joining the poles was vertical, in my first investigation it was 

 horizontal. 



First Series. 



Experiment 1. — The battery consisted of sixty-six Bunsen's 

 elements. Hard charcoal was used for the pole-points. When 

 the points were in contact and there was an inch of the rheostat 

 in the circuit, the tangent-compass indicated 56° 26', and when 

 there were 10 inches, 38° 30'. From this it may be calculated 

 that the entire resistance M besides the rheostat was =9*067, 

 and the electromotive force of the battery E was 15*171, that 

 force being taken as unity which produces a deflection of 45° in 

 the tangent-compass when the resistance was 1. 



When a luminous arc having a length of three divisions of 

 the scale was formed and an inch of the rheostat was interposed, 

 the deflection was equal to 37° 30'; and when 11 inches were 

 interposed it gave the same deflection. With the same inten- 

 sity there was obtained : — 



The resistance in the arc of two divisions = 8*9 

 „ „ one „ —7'7 



From this we obtain a = 6'6, and 5 = 1"15. The electromo- 

 tive force of the battery and the resistance in the conduction was 

 then measured afresh without the rheostat, and there was ob- 

 tained M = 8*527, and E = 15*73. During these experiments 

 the mean, therefore, was M = 8*797, and E = 15*451. 



We thus get D= *L =0*3334E = 5*15. 



Experiment 2. — Thirty elements. A distinct luminous arc 



* 



M. Lemstrom of Helsingfors took part in all the following experiments 



