THE STEEAMERS IN THE ELECTRIC SPARK. 83 



NOTES ON THE PHOTOGRAPHS. 



The following notes refer to the spectrum photographs reproduced in Plates 2 to 4. The earlier figures 

 in Plate 2 have already been described in the text. 



The capacity and inductance in the discharge circuit, given in the data for each spark, were measured 

 with a cymometer, and the period of the oscillations from the negatives themselves in terms of the speed 

 of the rotating mirror. The latter was measured before and after each spark by timing the rotations of a 

 lower wheel of the gearing, which rotated with the T J<y part of the angular velocity of the mirror itself. 

 The periods agree with their calculated values, as a rule, to within one or two per cent. Measurements of 

 the durations of the lines are recorded for the " inductionless " spark only, and they represent the times 

 from the beginning of the discharge during which a photographic effect is distinguishable on the negative. 

 The inductance of the connecting wires in the inductionless spark was also measured with the cymometer. 

 It amounted roughly to 25 microhenry. This would make the theoretical period of the oscillations from 

 0'3 to 0'46 micro-second according to the capacity in circuit. This is in itself inappreciable, but if there 

 were, say, a dozen oscillations the drawing out in the photograph would be easily observed. This is in 

 accordance with the photographs which show an appreciable broadening of the base of the metal lines in 

 most of the inductionless sparks, but this has not been deducted from the durations. 



The negatives were taken on Wratten's " Verichrome," in some cases on their " panchromatic " plates, 

 and the spectrum recorded extends from about A 6300 to A 3500. In the reproductions the top of each 

 figure always represents the red end of the spectrum. Only the stronger linen in the spectrum are 

 referred to in the lists below. The negatives reveal many of the weaker lines in the spectrum, most of 

 which have been identified, but as they do not show in the prints they are not referred to except in 

 exceptional cases. The streamers are often composite in character, a strong streamer being formed by the 

 overlapping of several weak lines close together. Groups of lines which combine to form a single set of 

 streamers are enclosed in brackets. 



The vertical ordinates are the same in all the photographs, but were reduced by the camera in the ratio 

 of 0'38. On the other hand, the photographs have been enlarged 1^ diameters in the reproduction, thus 

 the figures represent the relative lengths of the sparks, but a spark length 0'57 cm. in the figure 

 corresponds to an actual spark length of 1 cm. 



The velocities were determined by careful measurements on the negatives by a reading microscope of 

 the inclination of the streamers with respect to the air lines of the spark. They refer to the front edge of 

 the metallic vapour in the inductionless spark and to the centre of the streamers in the sparks with self- 

 induction. Numbers for the velocities are only recorded when the streamers were sufficiently clear in the 

 negatives to enable a fairly definite measurement of several streamers in the same spark to be made (the 

 detail of the streamers which permitted the measurements has sometimes disappeared from the repro- 

 ductions). The numbers given represent the average velocity over the first two millimetres of the path ; 

 the initial velocities are probably two or three times as great, but are very difficult to measure. 



Aluminium. 



Fig. 9. Capacity 0223 microfarad, no inductance, velocity 350 metres per second. 



Fig. 10. Capacity - 0223 microfarad, inductance 95 microhenries, period 8 -95 micro-seconds, velocity 

 650 metres per second. 



The only arc line of aluminium, the doublet (3961, 3943), shows plainly as lasting much longer than 

 any of the others, its duration in fig. 9 being 100 micro-seconds. In fig. 10 the streamers in it can be 

 distinguished, but the whole spark gap is filled with more or less light which lasts in the centre long after 

 the actual spark has ceased. The lines of next longest duration (20 micro-seconds) are the group of spark 



M 2 



