THE STREAMERS IN THE ELECTRIC SPARK. 85 



Calcium. 



Fig. 19. Capacity 0-0106 microfarad, no inductance, velocity 780 metres per second. 



Fig. 20. Capacity, 0-0223 microfarad, inductance 260 microhenries, period 15'1 micro-seconds, velocity 

 1 700 metres per second. 



Calcium is remarkable for both the great velocity and duration of its streamers. The lines visible in 

 the photographs are all prominent arc lines, and they give rise to a general blur which partly masks the 

 streamers even at high inductances. The only exception is the double line (3737, 3706), which has a 

 much smaller duration than the others. 



The four bands visible in fig. 20 can be identified in the negatives with the following lines or groups of 

 lines : 



(1) 5587, 5348, 5269. Duration 170 micro-seconds. 



(2) (4454, 4434, 4425), (4319, 4302, 4289, 4282), 4227. Duration 146 micro-seconds. 



(3) 3968, 3933. Duration 120 miero-seconds. These show the streamers plainly, although the 



spark gap is filled with a general light. 



(4) 3737, 3706. Duration 40 micro-seconds. These lines give quite sharp streamers which are 



overlapped by a general blur due to the strong arc line (3644, 3631, 3623), the difference in 

 the behaviour of the two lines being very apparent. 



Lea*/. 



Fig. 21. Capacity 0-0223 microfarad, no inductance, velocity 190 metres per second. 

 Fig. 22. Capacity 0'0223 microfarad, inductance 40 microhenries, period (i'3 micro-seconds. 

 Fig. 23. Capacity 0'0223 microfarad, inductance 260 microhenries, period 1;VO micro-seconds, velocity 

 400 metres per second. 



The following lines are visible : - 



Arc lines. Duration 115 micro-seconds. 

 (4062, 4058, 4019). 

 3740, (3683, 3671), 3640, 3572. 

 These form the two drawn-out bands in which traces of streamers can be seen in fig. 2.'! ; the 



individual lines cannot well be distinguished in the prints. 



Spark lines. Duration 16'5 micro-seconds. Sharp streamers repeated throughout the spark. 

 (5608, 5545), 5372. Very faint at the extreme top of the spectrum. 

 4386, 4246. The two sets of strong streamers just above the first arc band. 

 Condensed spark lines. 



(3854, 3842, 3833). The streamer corresponding to this unresolved group forms an intense 

 white dot in fig. 21, a streamer repeated once only in the negative of fig. 22, and it has 

 entirely disappeared in fig. 23. The three photographs show how very sensitive the lines 

 of this class are to the influence of self-induction. 



Magnesium. 



Fig. 24. Capacity 0-0106 microfarad, no inductance, velocity 450 metres per second. 



Fig. 25. Capacity 0-0223 microfarad, inductance 40 microhenries, period 6'0 micro-seconds, velocity 



800 metres per second. 



r 



Fig. 26. Capacity 0-0223 microfarad, inductance 170 microhenries, period 11 -5 micro-seconds, velocity 

 800 metres per second. 



See p. 77. The arc lines in the inductionless spark last 60 micro-seconds, the spark line 4481 only 

 15 micro-seconds. The anode discharge is very marked in magnesium, and at the beginning of the 



