CREW AND BASQUIN. — LUMINOSITY IN THE ELECTRIC ARC. 347 



represented by the bright triangular area in the first three photographs 

 of each series) persists for about xoucjo second after the brash has passed 

 on to the slate sector. But what remains longest of all is the detached 

 white cloud which persists from eight to ten times the whole interval 

 covered by one of these series of photographs. Each of the twenty-one 

 photographs is a composite of about 200 single exposures, the whole 200 

 exposures occupying just two seconds. 



In Figure 9 are reproduced seven series of four photographs, each series 

 representing the rise or growth of the arc. Each series is arranged in a 

 vertical column. The first photograph of the series, the one at the top, 

 was taken yoioo second after the circuit was closed, the next three are 

 made at immediately succeeding intervals of 50^00 second. These are 

 also composites of about 200 exposures, the whole set of 200 occupying 

 but two seconds. It will here be seen that the luminous cloud which was 

 so slow in disappearing is also tardy in making its reappearance. 



In the third and seventh series no arc appears even after xoioir second. 

 A bouncing of the brush might explain this for a single exposure. But 

 it is not so easily explained in a composite of 200 exposures. 



In Figure 10 are reproduced six typical photographs of the detached 

 cloud as it appears at the middle of the interval during which the current 

 is off the arc. The bright point to the right is the red-hot iron electrode. 

 The exposure is two seconds, aperture ^ the focal length — " Carbutt A " 

 plates — unit magnification. These six photographs are all taken in 

 the same phase. It will be observed that no trace of the arc remains, 

 but that the luminous cloud is as bright as red-hot, perhaps molten iron. 



Whether or not this after-luminosity is a phenomenon corresponding to 

 the " flame " which accompanies the carbon arc remains to be decided. 



III. Effect of Self-Induction. 



In order to discover whether self-induction had anything to do 

 with the persistence of this after-luminosity, the primary of a 25-light 

 transformer was put in series with the arc, the circuit being arranged as 

 indicated in Figure 7, so that the switch S could throw in or cut out the 

 transformer without interrupting the arc. No eflfect was certainly observ- 

 able either by the eye or on the photographs, although the time-constant 

 of the circuit must have been enormously increased. 



So far as the evidence thus far presented is concerned, this persisting 

 luminosity appears to be what Wiedemann would call cherai-luminescence, 

 or thermo-luminescence, and not electro-luminescence. 



