KENT. — SPECTRA OF THE ZINC ARC AND SPARK. 105 



thus show that with an alloy and inductance the structure is rendered 

 very simple and the light even more monochromatic than with the 

 lower voltage arc. 



Comparison of arc and disruptive spark. 



48 (c). 4G80. Pfund arc at low current shown by two openings 

 of the five opening shutter: end of image of a disruptive spark of 

 3 mm. gap with a 4 mm. auxiliary gap in series, shown by three 

 openings. Double-coated Seed, gilt edge 27 plate. Hydrochinone 

 developer. Exposure: arc, 15 seconds, spark 1 minute. Note that 

 there is structure in the spark and that it lies to the right, the 

 region of longer wave-lengths. See especially the middle of the 

 five shutter openings. This is confirmed by two other sets of ex- 

 posures. The reproduction is poor, owing to the fact that the 

 structure is not strongly marked, and is obscured by a continuous 

 pattern. 



Comparison of arc and inductance spark. 



85 (a). 4810. A comparison of an inductance spark outside 

 (inductance, three layers of coil (a); exposure, 30 seconds; and 

 center of gap) with Pfund arc inside (low current and exposure 5 

 seconds). Single order condition but with the stronger of the two 

 adjacent orders toward the violet. Notice that the maximum in- 

 tensity of the structure lies toward the red in the spark in comparison 

 with the arc. This is confirmed by another set of exposures in which 

 the arc was given relatively greater exposure time. Of course if 

 another part of the arc had by chance been used, the result might 

 possibly have been different. And again, greater arc current might 

 have made some difference in the structure and further, as the rapidly 

 fluctuating conditions in the arc change the structure, the distribu- 

 tion of energy might at another instant have been different. But 

 further exposures are confirmatory with respect to 4810, and show 

 a like phenomenon in the case of 4722 ; and still others are confirma- 

 tory with respect to 4722, and show a like phenomenon in the case of 

 4680. Further, two other sets of photographs taken some days later, 

 confirm these results for all three lines; and two more using carbon 

 terminals and a 3 amp. current show the same effects in all three lines. 

 And again four other sets taken upon still another day, with a 220 

 volt, 3.3 amp arc between carbon terminals, give in every case the 

 same results for these three lines. 



Such agreement proves that the effect cannot be fortuitous. How- 

 ever, as the inductance spark is steadier and easier to control, it is 

 well to compare sparks having different inductances in circuit: — 



