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VI. On the. Constitution of the Electric S}mrk. 

 Jfy AuTiiuu SCHUSTEII, F.It.S., and GUWTAV HBMHAU: n 



v 



Received February 2, Read February 2, 1899. 

 [PLATES 8-12.] 



1. Object of Research and Description of Apparatus. 



WHEN an electric spark passes between metallic electrodes, the spectrum of the metal 

 appears not only in immediate contact with the electrodes, but stretches often across 

 from pole to pole. It follows that, during the short time of the duration of the 

 spark, the metal vapours must be able to diffuse through measurable distance* 



The following investigation was undertaken primarily to measure this velocity of 

 diffusion, with the special view of comparing different metals and different lines of 

 the same metal. 



Dr. FEDDERSEN* published in the year 1862 an interesting research in which' 

 photographs of sparks were taken after reflection from a rotating mirror. He was 

 able to draw some important conclusions from his experiments, but it was necessary 

 for the more detailed examination we had in view, to analyse the light by a 

 spectroscope, so as to distinguish between the luminous particles of air and those of 

 the metal poles. 



Attempts to measure the required velocity, using rotating mirrors either between 

 the spark and the slit of a spectroscope or between the prism and telescope, were 

 made by one of us at various times during the last fifteen years. They failed because 

 the method requires that the spark should pass when the mirror is in the same 

 position, and no satisfactory device could be found to secure this object, without at 

 the same time complicating the spark circuit, which it was necessary to confine as 

 much as possible to the electrostatic capacity and spark gap. 



Instead of using a rotating mirror, it is possible to secure the same object by 

 taking photographs in a rapidly moving film. An arrangement of this kind was 

 employed by Professor H. DIXON in his experiments on explosions, and proved at 

 once successful. 



The principle of the method is extremely simple, and may be employed in all cases 



* ' Pogg. Ann.,' vol. 116, p. 132 (1862). 



4.12.99 



