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XXVII. Further Experiments on the Constitution of the Electric 

 Spark. By T. Royds, M.Sc, 1851 Exhibition Science 

 Scholar*. 



[Plate IV.] 



THE results obtained by photographing on a rapidly 

 moving film the spectrum of an oscillatory spark, in 

 the manner first employed by Schuster and Hemsalechtr 

 have already been published in a previous paper f. The 

 method has now been applied to study the spark when the 

 self-inductance of the circuit is gradually increased. The 

 light being weak when the self-inductance is large, it was 

 found necessary to remove the prisms, thus obtaining photo- 

 graphs similar to those of Feddersen and others. Though, 

 without prismatic dispersion, it is impossible to distinguish 

 between the different lines of the same element, it remains 

 possible to differentiate between the streamers of metallic 

 vapour and discharges through the air, as will be shown 

 below. It was thought that the experiments with the im- 

 proved definition might be of some value in studying the 

 effects of self-induction. 



The apparatus was the same as that used in the previous 

 experiments, and differed but little from the original one 

 employed by Schuster and Heimalech. Condensers whose 

 capacity amounted to J mfd. were charged from a large 

 Wimshurst machine and discharged through a spark-gap 

 8 mms. long between metallic electrodes. Self-inductances 

 ranging in value up to *034 henry could be inserted in the 

 circuit. An image of the spark was formed on the slit of a 

 collimator, and the slit-image focussed on the photographic 

 film by a third lens. The photographic film was carried on 

 a disk which was rotated by a motor. The velocity of the 

 photographic film was generally in the neighbourhood of 

 100 m./sec. 



Luminous Discharges through the Air. — The luminosity of 

 the air can readily be distinguished from that of the metallic 

 vapour by the fact that the former occurs almost simul- 

 taneously at all parts of the spark-gap, extending the whole 

 distance between the electrodes, whilst the latter starts from 

 the electrodes and is propagated with a measurable velocity 

 towards the middle of the spark-gap. 



* Communicated by Prof. A. Schuster, F.Il.S. 



t Schuster and Hemsalech, Phil. Trans. A. cxciii. p. 189 (1899). 



X Royds, Phil. Trans. \. cciii. p. 333 (1908). 



