The Constitution of the Electric Spark. 331 



" The Constitution of the Electric Spark." By ARTHUR SCHUSTER, 

 F.K.S., and G. HEMSALECH. Eeceived January 24, Eead 

 February 2, 1899. 



(Abstract.) 



When an electric spark passes between metallic electrodes, the spec- 

 trum 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 distances. 



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. 



Feddersen published, in the year 1862, an interesting research, in 

 which photographs of sparks passing between different metal poles 

 are taken after reflection from a rotating mirror. He could from his 

 experiments draw some conclusions which have a bearing on the subject, 

 but it was necessary for our purpose that the light should also be sent 

 through a spectroscope, so as to distinguish between the luminous 

 particles of air and those of the metal poles. 



The method of the rotating mirror tried during the course of several 

 years in various forms by one of us, did not prove successful. On the 

 other hand good results were obtained at once on trying the method 

 used by Professor Dixon, in his researches on explosive waves. This 

 method consists in fixing a photographic film round the rim of a 

 rotating wheel. All that is necessary for its success is to have sparks 

 so powerful that each single one gives a good impression of its spec- 

 trum on the film. Were the sparks absolutely instantaneous, the 

 images taken on the rotating wheel would be identical with those 

 developed on a stationary plate, but on trial this is found not to be 

 the case. The metal lines are found to be inclined and curved when 

 the wheel rotates, and their inclination serves to measure the rate of 

 diffusion of the metallic particles. The air lines, on the other hand, 

 remain straight, though slightly widened. 



To avoid the tendency of the film to fly off the wheel when fixed 

 round its rim, as in the original form of the apparatus, a spinning disc 

 was constructed for us by the Cambridge Scientific Instrument Com- 

 pany. The film is placed flat against the disc, and is kept in place by 

 n second smaller disc, which can be screwed lightly to the first. The 

 diameters of the two discs are 33 and 22 '2 cm., the photographs being 

 taken in the annular space of 1O8 cm., left uncovered by the smaller 

 disc. An electric motor drives the disc, and we have obtained velocities 

 of 170 turns per second, though in our experiments the number of 



