THE STREAMERS IN THE ELECTRIC SPARK. 77 



When the metal lines are close together a certain amount of overlapping still takes 

 place, but it is generally possible to distinguish the streamers due to the stronger 

 lines even when this occurs. 



Using the spark itself as the source of light instead of its image thrown on the slit 

 of the spectroscope has the advantage not only of simplicity, but also of giving us a 

 truer rendering of the course of the streamers. The course of the spark discharge is 

 often very variable ; instead of going straight across from pole to pole, the spark 

 curves round, and its image is very likely to fall off the slit in the centre of the gap ; 

 and with a slit in such cases the resulting photograph will give a false impression ot 

 both the velocity and the extent of the streamer. By using the spark itself as the 

 source of light, the air lines show the actual path taken by the discharge, and any 

 curvature in them can be taken into account in studying the shapes of the streamers. 



With this apparatus, and using the electrical arrangements previously described, 

 about a hundred exposures have been made on sparks with different metals, 

 inductances, capacities, and spark lengths. From the resulting photographs, of which 

 a selection is reproduced to accompany this paper, the behaviour of the different 

 lines, as regards the appearance, velocity, duration, &c., of the streamers, could be 

 studied at leisure. Descriptive notes on the photographs reproduced are given at 

 the end of the paper, and in the following account I shall confine myself to describing 

 the general conclusions which follow from the examination and comparison of the 

 whole series, referring when necessary to the figures in which the best illustrations 

 of them can be seen. 



Durations of Lines. 



The durations can best be studied quantitatively when there is no inductance in 

 series with the spark gap beyond that of the connecting wires. The period of the 

 oscillations is then extremely small, and the whole spark from an electrical point of 

 view is over before the image is appreciably drawn out on the plate. The fact that 

 nevertheless the metal lines are always drawn out shows that their vibrations last an 

 appreciable time after the stimulus which has excited them has ceased ; this time is, 

 moreover, very different for the different lines. 



We may take as an example the magnesium spark, fig. 24 (Plate 3), the luminosity 

 of the triplets (5183, 5172, 5167) in the green (nearest the top),* and (3838, 3832, 

 3829) in the ultraviolet (both unresolved of course), lasts at' least four tunes as long 

 as that of the line A. 4481, which itself has a duration of 16 micro-seconds after the 

 actual spark has ceased. The two triplets are prominent arc lines, 4481 is the well- 

 known spark line, absent from the arc under ordinary conditions. When a little 

 inductance is inserted in the sparking circuit, as in fig. 25, the difference in the duration 

 of the lines makes itself evident in another way. With X4481 the streamers are 



* The line has almost disappeared in the reproduction, but in the negative shows as an exact but fainter 

 copy of the ultraviolet line at the bottom of the figure. 



