by which a Galvanic Luminous Arc can be produced. 44<7 



Experiment 16. — This was made immediately after the last ; 

 the battery consisted of fifteen elements, with which, how- 

 ever, no luminous arc could be produced. M was =0*86, and 

 E = 255. 



Fifth Series. 



Silver, said to contain 10 per cent, of copper, was used for 

 the pole-points. 



Experiment 17. — The same sixty elements were used for the 

 battery as formerly. On measuring, M was found =8*02, and 

 E = 12'75. 



When two inches of the rheostat were introduced into the cir- 

 cuit, and the length of the luminous arc was three divisions, the 

 tangent-compass was at 43°. With the same intensity we had : — 



The length of the arc 3 divisions, its resistance = 4*4 



)) >> 1 }J )} = 0'0 



From this we calculate # = 2' 75, and 6 = 0' 55. 



The resistance and the electromotive force were next deter- 

 mined, and there was obtained M = 8'67, and E = 14-03. Hence 

 in the mean we had the values M = 8"34, and E = 13-39. 



From this we find D = ^- x E = 0-1865E = 2'5. 



14*74 



Experiment 18. — Fifteen elements were used for the battery, 

 which gave a light when the pole- points were suitably shaped. 

 In this case M = 092, and E = 2-64. 



Experiment 1 9. — In this experiment twelve elements were used. 

 A luminous arc was only occasionally produced, and especially 

 when the poles had the shape which had been imparted to them 

 by a powerful current. The measurements gave M = 0*72, and 

 E = l-89. 



4. It is seen, from what has been said, that M and E varied 

 to no inconsiderable extent even in one and the same experiment. 

 Hence it was not possible to determine with the desired accuracy 

 the value of D. Meanwhile these values are not to be looked 

 upon as too untrustworthy, — partly because the resistance and 

 the electromotive force of the battery were determined both before 

 and after the experiments which were made for the determination 

 of D, and therefore their mean may be regarded as valid for the 

 time in which the latter operations were made; partly also 

 because M and E, with the exception of Series 1, both increase 

 simultaneously, owing to which their variations, in the determina- 

 tion of D, partially compensate one another. In seeking the 

 smallest value of E with which a luminous arc could be produced, 

 a great number of observations was always made. Although 

 several successive experiments failed in producing a luminous 

 arc, it occasionally happened that it could be done by repeatedly 



