306 Occurrence of Enhanced Lines in the Arc. 



the metal pole. Again, if air is blown through the conduct- 

 in op vapours o£ the arc and their resistance thereby increased, 

 spark lines are brought out prominently even with a carbon 

 cathode. But the emission of spark lines by the arc is 

 strongest when the formation of a proper arc, and, therefore, 

 a rapid ionization of the medium, is prevented or, at all 

 events, retarded by means of a liquid which fills the arc-gap. 

 The flashing up of spark lines at the moment of striking an 

 arc between cool metal poles in air may be accounted for by 

 the scarcity of conducting vapours at this early stage ; for 

 as soon as conducting vapours are formed in abundance re- 

 sulting in the establishment of a steady arc with rise in 

 temperature and increase in electrical conductivity (partly, 

 perhaps, as a result of chemical reactions between the metal 

 vapour and the constituent gases of the surrounding atmo- 

 sphere), the <= park-lines either disappear from the spectrum or 

 show only as traces in the vicinity of the poles. The greater 

 prominence of the spark lines when the arc is taken in 

 hydrogen, may be due perhaps to the relatively small amount 

 of chemical reaction which takes place between the metal 

 vapour and this gas. In short, the whole of our experimental 

 evidence points to the fact that spark lines are emitted when 

 electric currents are passed through media (vapours or gases) 

 which possess a low degree of ionization. Since such a pro- 

 cess involves the application of powerful electric forces 

 givino- rise to the establishment of high potential gradients 

 in the medium concerned, we are finally led to conclude that 

 the emission of spark lines is' connected with the existence of 

 strong electric fields. A simple consideration shows indeed 

 that for small pole distances the electric field within the arc- 

 o-ap can be very great even with a small potential difference. 

 Thus, for a pole distance of 0*05 mm. the intensity of the 

 electric field with an applied potential of 110 volts is equal 



to 22,000 . As we have shown in the course of this 



research spark lines are strongly emitted by the arc only 

 whilst the distance between the electrodes is precisely very 

 small. 



Manchester and Paris, 

 May 1921. 



