Spark line structure as a function of the inductance.

78 (c). 4810. Between single and double order condition. Center of 1.5 mm. gap. Three layers of inductance coil (a). Exposure: 1 minute. Two main components. Note that the right component of the quadruplet is as strong as, or stronger than the left, when the position of orders is such that it would be weaker. This fact is confirmed by other exposures.

79 (e). 4722. Between single and double order condition. Electric conditions as in 78 (c). Exposure: 1 minute. Shows three main components.

80 (a). 4680. Between single and double order condition. The electric conditions are as in 78(c) and 79(e). Two main components. Exposure: 1 minute.

65 (b). 4810. Single order condition. Center of 4 mm. spark gap under different conditions. Outside, no inductance, 5 seconds: inside coil (b) in circuit, 45 seconds. Notice the two side components in the inductance spark image.

68 (a). 4722. Single order condition. Electric conditions, similar to 65. Note inequality of intensity of inductance line components. Exposures: 30 seconds with inductance and 3 seconds without. Single order condition. Center of a than 2 mm. Three, two and one layers of

94 (c), (d), and (e). 4810. very small gap - less coil (b), respectively.

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96 (c), (d), and (e). 4680. Double order condition. Same set of operations as in 94. Notice in both plates a continuous increase of intensity of the old components lying toward the red and the development of new ones as the inductance is decreased. Another photographic plate (numbered 95) clearly confirms this for 4722. On all three, 94, 95 and 96, there were also taken shutter comparisons showing the relative positions of the components given with one, two and three turns. These all show that the component coming up with decrease of inductance is the one toward the red: the component toward the violet retains its position while its intensity becomes relatively less. The effect of removal of inductance is similar to that obtained by moving up to the end of a somewhat longer gap leaving the inductance the same. (See 111b).

The conclusions to be drawn from the photographic study are:1. That it is impossible by means of the echelon grating to compare the positions of maximum density of any but quite monochromatic sources, whether the condition be either double or single order.

2. That it is impossible in general to distinguish the images given

by a Nernst lamp, an arc of great vapor density, and a highly disruptive spark between terminals of the pure metal.21 These sources give, in fact, nothing but the so-called "diffraction" as distinguished from the "interference" pattern.

3. That inductance, even in small amounts, is extremely efficient in reducing the intensity of the continuous or diffraction pattern and producing structure in the spark image.

4. That the structure varies with the part of the inductance spark image used whether end or center; the end showing an enhancement of the intensity of the components lying toward the red.

5. That as the value of the inductance is increased, the red components in the structure become less intense.

6. That even a disruptive or non-inductance spark between brass terminals shows structure in the zinc lines studied and that, if in addition inductance be inserted, the resultant lines are as sharp, or even sharper, than those given by a low current arc.

7. That a small amount of vapor in the arc, even with fairly high current (e. g. 8 amp.) produces conditions favorable to structure other than the fluting which occurs when the arc is heavily charged with vapor and is noisy.

8. That on all plates obtained upon which the positions of the components of the spark with small inductance are compared with the positions of the components of the arc at low current (about 3.3 amp.) the center of gravity of the spark structure lies further toward the red than that of the arc.

GENERAL CONCLUSIONS.

That conflicting results were obtained by Janicki and Nutting is probably due to the fact that different sources of light were employed. The structure Nutting describes is unquestionably real. Certainly echelon gratings may give ghosts. That the Petitdidier instrument used in this investigation is free from such, is shown by the fact that the green line of mercury shows no false lines.

Further, from the visual observations made upon arc lines, it is perfectly clear that the "ghost" argument will not explain the endurance of a satellite or its increase in intensity, when a formerly brighter line grows fainter or disappears entirely, nor, specifically,

21 This is true of the spark only when the echelon is not powerful enough to resolve the components of the fluting.

a case such as that recorded on page 101 under Zn 4722 at 8.8 amperes. It is impossible for the main line to disappear and the ghost remain; and again, even if ghosts were present, there is no reason why these should appear in the case of any one line with the spark as a source, and not with the arc. The presence of neither a symmetrical nor unsymmetrical ghost structure could produce the enhancement of the red satellites in the spark.

A certain objection may, however, be made: namely, that the presence of the diffraction pattern between the orders when the instrument is in a double order condition, might cause satellites which are of low intensity to appear (when otherwise they would not) in much the same manner as fogging a photographic plate will carry the exposures of "low lights" up along the intensity curve so that they will become visible.22 In response to this objection, it may be said that the satellites in question are not always of low intensity, either visually or photographically; and they even come up on the right side when the diffraction pattern lies to the left.

We must conclude, then, that there exists for some unknown reason a fairly progressive increase in the intensity of the red satellites of these three zinc lines with decreasing inductance. There follows at once the unsymmetrical broadening to the red of the images given by instruments of less resolving power, namely, prism or grating spectro

scopes.

The unsymmetrical satellite system may be produced by the high potential gradient in the spark; why, the writer, of course, cannot state. Disruptiveness is not a determining factor, for in the same spark we obtain from different parts of the gap different line structure. Vapor density probably does not of itself determine structure, but may influence the potential gradient. In the arc high density seems to produce a tendency toward complexity of structure, but not an asymmetry of a regular or enduring type.

23

All the writer's observations, both visual and photographic, confirm the results obtained by Nutting, dealing with arc structure. The results of this study also confirm the shifts found by the writer to exist at lower dispersion, shifts, great at the end of a fairly large gap of a non-inductance spark between terminals of the pure metal, lessened or removed entirely by the addition of inductance, and by the use of the central region of the gap; and lessened also by the use of an alloy. In this former work the standard of reference employed

22 R. W. Wood actually used this method.

23 Astrophysical Journal, 22, No. 3, Oct. (1905).

was a carbon arc of somewhat greater current than here used, but the amount of vapor was never great, only small bits of metal being inserted in the arc, and the exposure always being made when it was burning quietly. These two sets of standards were probably much the same. Still, assuming them different, if the potential gradient determine the enhancement of the red satellites and we accept Nuttings classification of gradient, from low to high the order being, (1) heavy current arc, (2) low current arc and inductance spark, (3) high capacity and non-inductance spark, then the assymmetry of satellites (and resultant shift) obtained in this investigation with low current arcs as standards would be even less than that found with the somewhat higher current arcs previously used. However, as stated above, in the arc there seems to be no regular, controllable nor enduring enhancement of either red or violet satellites.

Janicki's suggested explanation of the shifts obtained - namely, as "unsymmetrical reversals like those of chromium and calcium, reversals which their grating would not resolve and which appeared to them as line-shifts" must then be replaced by this enhanced satellite theory.

The distances between the satellites in Plate 2, 48 (c) are approximately 0.05 Ångstroms. We may then say that the removal of two layers of inductance in coil (a) has shifted the center of gravity of the line at least 0.02 Ångstroms. In the extreme case then, with no inductance in the circuit, the shift might easily be in the neighborhood of 0.032 Ångstroms, as formerly obtained.

The writer wishes to record his appreciation of the kindness of Professor Goodwin of the Massachusetts Institute of Technology in loaning his Petitdidier echelon. To the Rumford and Bache Committees, and a personal friend, Mr. J. DeL. VerPlanck, the writer is indebted for funds which made this investigation possible. In the actual work of obtaining the results he wishes to acknowledge the faithful assistance rendered by various students, especially Messrs. Walter F. Burt, Russell T. Hatch, Charles H. Smith and Carl K. Springfield.

PHYSICS LABORATORY, BOSTON UNIVERSITY,

JUNE, 1912.