the Occurrence of Enhanced Lines in the Arc. 843> 



§ 5. Effects of gaseous media. 



The gases which were used in these experiments may loe 

 divided into two groups — namely, those in which a steady 

 arc can be established between metal electrodes, such as air 

 and nitrogen, and secondly those in which under similar 

 electrical conditions only short arc flashes can be obtained,, 

 such as hydrogen, oxygen, and coal-gas. It seems likely 

 that the kind o£ arc produced in each case is in great 

 measure dependent upon the nature and intensity of the 

 chemical reaction which sets in between the vaporised 

 electrode material and the surrounding gases. 



An ordinary seven-ampere arc burning steadily between 

 magnesium electrodes in air emits the spark line X 4481 

 feebly at the cathode and a little more strongly at the 

 anode. Along the path of the arc only the merest traces 

 of it can be detected. If, however, the observations are 

 begun at the moment of striking the arc the spark line 

 is seen to flash up intensely just when the electrodes 

 separate, and it fades away again as the arc becomes 

 established. Photographs taken of the initial stage of an 

 arc between cadmium poles show clearly that the intensity 

 of the spark lines falls off abruptly after the upward- 

 moving electrode has passed through only a very small 

 distance. In fact both the order of evolution and the 

 general character of the spectral phenomena which mark 

 the early stages of development of the electric arc in air 

 are very similar to those observed with liquid films. In 

 order to account for the strong emission of spark lines- 

 during the initial phase of an arc in air, we venture to 

 suggest the following explanation. As the upper electrode- 

 recedes from the stationary one a more or less perfect 

 vacuum is momentarily left between them. It is through 

 this vacuous space or film of rarefied air that the initial 

 stages of the discharge take place. As no air is present 

 at first to start chemical reactions between it and the metal 

 vapour, both conductivity and temperature of the medium 

 remain relatively low, so that the effect of strong electric 

 forces may develop without impediment. As soon, how- 

 ever, as air has penetrated the vacuous space chemical 

 reactions set in which, by causing increase of both tempe- 

 rature and conductivity of the medium, lead to the establish- 

 ment of a stable arc. If a current of air is blown through 

 the arc from its commencement, ionization of the vapours is 

 rendered more difficult and the brighter spark lines remain 

 visible almost throughout the whole length of the arc flash 

 during the second phase. 



