Dr. P. L. Rijke on the Inductive Spark. 443 



the other with negative electricity*. As to the intermediate 

 parts, they are also electrified ; and although we are unacquainted 

 with the precise law that connects the degree of electric tension 

 with the distance from the middle point, we know at least that 

 these magnitudes increase together. It is, then, evident that when 

 the inductive wire is discharged, the electric charges of the two 

 extremities first unite, and the charges from the parts nearer 

 and nearer the centre follow in succession. Now the fluids con- 

 tained in the two extremities obviously unite in the form of an 

 ordinary spark ; but the same cannot be the case with the charges 

 of the parts nearer the centre. In fact the experiments of Mr. 

 Wheatstonet show that when two electric fluids, before uniting, 

 have to traverse a metallic wire T J jth of an inch in diameter and 

 half a mile long, the duration of the spark is greatly increased, 

 even to a degree that cannot be accounted for by the mere time 

 that electricity would require to traverse half a mile of such wire, 

 which proves that this prolonged duration of the spark is to be 

 attributed to that particular property of matter that physicists 

 have agreed to call electric resistance. Now if a wire of y^th 

 of an inch diameter, and only half a mile long, sensibly increases 

 the duration of the spark, what must be the effect of the second- 

 ary wire of Ruhmkorff' s apparatus, the resistance of which is 

 much more considerable ? We can form some estimate of it by 

 the time taken, according to the experiments of M. W. Weber J, 

 by an electric battery to discharge itself when the current has to 

 traverse a wet hempen cord. It is therefore evident, according 

 to my view of the subject, that the electric spark ought to have 

 some duration j and we know from an experiment of M. Lissa- 

 jons§, that when the inductive spark is observed in a mirror 

 which is at the same time shaken by the hand, the luminous 

 atmosphere presents the appearance of an elongated band, of 

 which the true spark occupies the posterior extremity, — thus 

 showing that the atmosphere in question really does endure for 

 some fraction of a second. 



5. We know that the appearance of the electric spark is gene- 

 rally changed when the two fluids, before uniting, have under- 

 gone any considerable resistance. Its colour is then altered, it 

 becomes tinged with violet and blue, while its illuminating power 

 is much diminished ; at the same time its form is changed, its 

 volume augmented, and it acquires the power of inflaming bodies 



* In the first machine constructed by Ruhmkorff, free electricity was 

 only found at one extremity of the wire ; this, however, was only in con- 

 sequence of a defect in the insulation, that M. Ruhmkorff has since remedied. 



t Phil. Trans. 1834. 



X Electrodynamische Maasbestimmungen, p. 295, 



§ Comptes Rendus, f vol. xlix. p. 1009. ' i 



2G2 



