Prof. E. Edlund's Investigation of the Electric Light. 117 



= Tg. 52° 29'; and when 13 inches was inserted, = Tg, 35° 42'. 

 From this are obtained M' = 7*08, and E' = 14*43. 



When 1*4 inch of the rheostat formed the resistance, and the 

 luminous arc was six divisions long, the magnetic needle indicated 

 44° 30'. With this intensity the following results were ob- 

 tained : — 



Length of the arc = 6 divisions. Resistance = 6*2 

 4 — 5-Q 



a » " a >) = 5'0 



From this we obtain « = 5 - 3 and £ = 015. 



The electromotive force calculated from this is found to be 



5'3E _' 

 D ~ ,14-68 - * 1 ' 



As 4*45 and 5*21 are equal within the limits of possible errors 

 of observation, we find from this series also that D is indepen- 

 dent of the electromotive force of the element. 



10. It follows from the above that D is smaller if copper is 

 used for the pole-points than when they are formed of solid 

 and hard carbon. This may arise from the circumstance 

 that the copper before being detached is in a fused or half-fused 

 condition, owing to which the work which the current performs 

 in the disintegration is less. Experiments were also made with 

 carbon of very porous structure; but, as no constant results 

 could be obtained with them, I do not think them worth com- 

 municating. Yet they showed that with the less-dense kinds of 

 carbon D was smaller. The experiments made with tin gave 

 also a smaller value for D ; but the results obtained with this 

 metal were less certain, because the quantity of oxide of tin 

 formed disturbed the steadiness of the light and caused irregu- 

 larities in the intensity. 



Since the two points have different temperatures, it was pos- 

 sible that a thermo-electric force might be at work in the lumi- 

 nous arc. Probably also the aqueous vapour in the air was de- 

 composed by the voltaic current ; and if the products of decom- 

 position (hydrogen and oxygen) were deposited on the poles, this 

 would give rise to a polarization current opposite in direction to 

 the principal current. But even if both these sources of electricity 

 are present in the luminous arc, they can only form an insig- 

 nificant fraction of the above-found electromotive force D, even 

 supposing the polarization to have reached its maximum inten- 

 sity. To get rid of all polarization, the experiments would have 

 had to be made either in a vacuum or in a gas which is not de- 

 composed by the current. 



