840 Mr. Gr. A. Hemsalech and the Comte de Gramont on 



two distinct phases, the durations of which are largely- 

 controlled by the speed of the upward-moving electrode 

 and by the thickness of the liquid film at the moment of its 

 rupture. Further, during the course of development of this 

 arc phenomenon certain spectrum lines stop or diminish in 

 intensity quite abruptly at the end of the first phase, others 

 suddenly cease to be emitted at some definite point during 

 the second phase, whilst some again continue till the end of 

 the luminous flash. It seemed to us that a more precise 

 knowledge of the exact positions of the various levels up to 

 which particular radiations are emitted might possibly be o£ 

 help in revealing the nature of those factors which govern 

 the production and development of the liquid film arc. 

 Accordingly measurements were made of the thickness 

 of the liquid film and of the relative altitudes of those 

 points which mark certain abrupt changes in the emission 

 of a few characteristic lines. The maximum duration of 

 the luminous arc flash was considered to be given by the 

 length of the arc lines, such as X 5184 of magnesium. These 

 measurements were made on the spectrograms and reduced 

 to actual value by allowing for the magnification of the 

 spectrum image, which in our case was equal to 1*2 times for 

 the green and blue regions of the spectrum. The following 

 values were obtained for magnesium with films of various 

 liquids and solutions, assuming that the thickness of the 

 film corresponds to the height of the spectrum in the first 

 phase : — 



Height from stationary 

 Nature of Thickness of electrode of 



liquid, film. liquid film. ^ A ^ 



X4481. X5184. 



f Glycerin 0*12 mm. 0*95 mm.* 277 mm. 



Paraffin oil 0'48 „ T04 „ t 67 „ 



Water 010 „ l\ r >l „ 4-76 „ 



Arc 



Magnesium sulphate 



at ■{ solution between \ 020 ., 1 67 „ 2-30 



80 volts. Mg poles j 



Magnesium sulpnate j 



solution between - 15 ,, 1'54 ,, 1*94 



^ graphite poles .,, ) 



f Water 009 „ 4-33 „ >66 



J Magnesium chloride } 



solution between [ 0-08 „ 2'40 „ 3'09 



Arc 



at 



onr . -., I solution between J- 

 200 volts. [ graphitepoles mmm ) 



* Continues as a narrow line throughout flash. 



t After the abrupt drop in intensity at this point the emission continues as 

 a faint narrow line up to 3'98 mm. 



Thus, with the exception of the paraffin-oil arc, the 

 thicknesses of the films are all practicall} r of the same 



