the Occurrence of Enhanced Lines in the Arc. 867 



§ 11. Summary. 



1. The emission of spark lines during the first phase o£ 

 a liquid film arc is independent of the nature of the 

 liquid. Further, the liquid film does not play the role of 

 a dielectric since a well conducting electrolyte acts in the 

 same way. § 2. 



2. The raising of the voltage of the arcing current 



O <5 O 



prolongs the life of the spark lines during the second phase 

 and, in the case of magnesium, brings into prominence the 

 lines of the Rydberg series. § 3. 



3. A remarkable behaviour is shown by the spark line 

 X 4481 of magnesium. The emission of this line either falls 

 off quite suddenly in intensity or stops abruptly at a definite 

 point during the second phase, as though some factor, abso- 

 lutely essential for its excitation, had suddenly ceased to act 

 or reached some critical minimum value below which it is no 

 longer capable of stimulating this particular radiation. It is 

 shown that this critical point corresponds to an applied electric 

 field of approximately 500 - — . §§ 3 and 10. 



4. The striking behaviour of the 3rd and 4th type en- 

 hanced lines of cadmium has disclosed the interesting fact 

 that during the gradual decline of the acting electric forces 

 the character of the spectrum changes quite abruptly at several 

 definite stages, as though the radiating centres concerned 

 were at these points undergoing sudden modifications in their 

 structure. § 4. 



5. Measurements of the thickness of the liquid film at the 

 end of the first phase together with an estimate of the speed 

 of the upward moving electrode have shown that some of 

 the phenomena occurring during the early stages of the first 

 phase are of the same order of duration as the oscillation 

 stage of a capacity spark in which spark lines are so strongly 

 emitted. §4. 



6. An explanation has been offered to account for the 

 flashing up of the spark lines at the instant of striking the arc 

 in various gaseous atmospheres. It is suggested that, as the 

 upper electrode moves away from the stationary one, a vacuous 

 space is momentarily left between them through which the 

 initial stages of the arc discharge take place. In this way 

 the electric field set up is enabled to develop its effect before 

 any appreciable chemical reaction has set in between the 

 vapours and the surrounding gas. § 5. 



7. A stable arc is obtained between magnesium poles in an 

 atmosphere of nitrogen showing X4481 as a sharply defined 



3K2 



