the Occurrence of Enhanced Lines in the Arc. 841 



-order of magnitude. Those obtained with the higher 

 voltage current seem to be the thinnest : this may not 

 merely be an accident, for it is quite conceivable that the 

 application of a higher voltage would tend to accelerate 

 the destruction of the film. Further, the results for the 

 line A, 4481 show clearly that the higher voltage, while 

 reducing the extent of the first phase, raises the critical 

 level of the spark line. The last column gives the length of 

 the arc flash. 



Bather suggestive results were obtained for various types 

 of cadmium lines with an 80 volts water arc, as shown by 

 the following values : — 



Thickness of film O09 mm. 



Heights from stationary electrode : 



Arc line A 4413 = 5*05 „ 



3rd type enhanced line 4416 = 2*41 ,, 



1 coco _ 1-9R i These lines continue to be 



Spark lines I r, rQ ~ l 99 " [ emitted faintly, and stop 



,- . -,-, , . M '"" l ^ " f abruptly at a height of 

 (first sudden drop m J j abou £ 2 ? 4 mmi 

 intensity) 



These results point to the existence of four distinct stages 

 in the spectral development of the cadmium arc flash, 

 namely : (1) the film stage, during which the spark lines 

 are enhanced and symmetrically broadened ; (2) a short 

 region of the second phase marked by a strong emission of 

 the spark lines ; (3) a stage during which the spark lines 

 are but feebly emitted and at the end of which both their 

 emission and that of the enhanced line X4416 cease 

 abruptly ; and (4) the last period of the flash, during which 

 • arc lines are alone emitted. These facts seem to indicate 

 that the emission centres, which during the first phase 

 emit strongly all vibrations, undergo sudden changes in 

 their constitution at several definite stages in the course 

 of the gradual decline of the forces acting upon them. It 

 is remarkable, although perhaps quite in keeping with the 

 interdependance between the structure of the radiating 

 atom and the character of the spectrum, that the abrupt 

 stoppage of the faint portions of the spark lines X5338 

 and X 5379 should coincide almost exactly with the equally 

 sudden cessation of the enhanced line X4416. 



The determination of the duration of the various phases 

 connected with the liquid film arc requires a knowledge of 

 the velocity with which the upper electrode moves away from 

 the stationary one. We have made no accurate measurement 

 of this quantity, but a rough estimate of the upward motion 



