A STUDY WITH THE ECHELON SPECTROSCOPE OF CERTAIN LINES IN THE SPECTRA OF THE ZINC ARC AND SPARK AT ATMOSPHERIC PRESSURE. BY NORTON A. KENT. Presented by Charles R. Cross. Received June 19, 1912. IN November, 1907, the writer published, in collaboration with one of his graduate students, an article1 attempting to meet certain objections made by Keller to the method of procedure adopted by the writer in certain former work 3 upon the question of the relative wave-lengths of certain lines in the spectrum of titanium and zinc as developed by the arc and spark discharge in air at normal pressure. That displacements of the spark lines to the red from the position of the corresponding arc lines actually existed on the photographic plates obtained, is regarded by the writer as unquestionably proven. It is certain, also, that the displacements were not due to any incorrect experimental procedure. It appeared to be worth while to study the matter further, seeking to ascertain, if possible, the cause of these displacements. As the echelon spectroscope had revealed structure in the lines of metallic spectra both in Plücker tubes and in the arc in vacuo and at normal atmospheric pressure, it seemed advisable to use this instrument to study the spark, noting the change in the form of the image as a function of the constants of the electric circuit. The titanium lines XX 3900 and 3913, formerly studied in detail, presented difficulties because of their short wave-lengths; therefore, it appeared best to concentrate the work upon zinc. A brief survey of the most important results in the case of this metal recently obtained by various observers is thus in order. 1 These Proceedings, 43, No. 11, Nov. (1907). 2 Ueber die angeblicke Verschiebung der Funkenlinien, Inaugural Dissertation, Christian Keller. 3 These Proceedings, 41, No. 10, July (1905). 4 Janicki, Annalen der Physik, 19, 36-79, Jan. (1906). Nutting, Astrophysical Journal, 23, No. 1, Jan. (1906). Nutting, Bulletin Bureau of Standards, 2, No. 3, Dec. (1906). HISTORICAL SURVEY. Houston who notes the changes which take place in the reversal system as seen by an echelon when a zinc arc "begins to hiss," speaks of the "striking forms of reversal," the distances between the different components in the line varying in the different parts of the arc. With one of his arcs and a small amount of vapor, he obtained the three blue lines of zinc "without reversals." Under certain conditions the three blue lines were "all doublets with components of equal intensity." Janicki 6 in his inaugural dissertation (1905) states that "an examination by the echelon of the lines of the zinc spectrum developed in a capillary tube of 0.3 mm. diameter with external electrodes at a temperature of about 460° showed them to be single lines." Nutting, in a paper on line structure, mentions the fact that Plücker tube spectra of rarefied gases moderately excited show narrow lines of the simplest structure, "but with a heavy current or capacity in parallel, if the pressure be greater than 3 or 4 mm. the lines broaden, and finally, with a spark in series with the tube, widen into a continuous spectrum, with the peculiar fluted appearance characteristic of spark lines." He states further that "sparks between metallic electrodes give lines far too broad for use as monochromatic sources. They are never less than half a tenth-meter broad. The effect appears to depend chiefly upon the amount of capacity used, and is greatly heightened by the use of another spark in series; that is, it is due to the steepness of the wave-front of the current wave.8 Inductance weakens the wings produced by capacity, and sometimes channels them, but never reduces a line to a simple structure. Occasional lines will appear to simply broaden out under the violence of the discharge, but ordinarily it is simply a case of the dark background-between spectra of different order becoming luminous." “Using a small current (0.02 amp.) of low voltage (5000) and low frequency (60) and a minimum of capacity, and electrodes of iron and brass, the spark lines were found to be still broad and diffuse. Lines due to impurities (sodium, for example) occasionally appear 5 Philosophical Magazine, 7, May (1904). 6 See Annalen der Physik, 19, 36-79, Jan. (1906). fairly sharp on but a faint background, but a number of tests indicated that it is impracticable to obtain narrow lines by introducing impurities into the spark." Further, when discussing arc spectra in general, he writes: "The structure which a line exhibits depends primarily upon its intensity; that is, upon the amount of a substance vaporized and the intensity of its excitation in the arc"; and specifically, in the case of zinc: "All four zinc lines are rather diffuse, and are usually found double or triple.*** The blue lines, 4810, 4722, 4680, are broad and diffuse, and show a trace of structure on reversal." In a general discussion attention is called to the fact that the structure of any one line is very variable, so much so that "we may hardly speak of any line as having a fixed definite structure, even with a minute specification of conditions of production." Types of lines are classified according to structure and behavior, and the general conclusion drawn that to explain certain types — lines which, when single, under some conditions become double or triple, symmetrically or unsymmetrically, with receding components of various relative intensities - the old absorption theory of reversal is not satisfactory.9 In another paper 10 covering the results of a search for intense and yet "pure" light standards, Nutting, sketching the development of the typical normal line in either the open air arc or at pressures less than atmospheric, states:-"with increase of intensity the line broadens, and finally separates into two; *** with further increase the two components continually broaden and separate"; and of highest "rank as to purity are the composite lines produced in the vacuum tubes measured between extreme components." 11 In a paper on relative intensities of spectrum lines an attempt is made to show that the changes produced in spectra by varying current, capacity, inductance, temperature and pressure, may be accounted for by a single variable, or at most, two. He writes: 12 "Several years ago the writer gave the steepness of the wavefront through a gas as condition for the preponderance of the secondary over the primary spectrum. Crew 13 almost at the same time con 9 Nutting advances a theory of broadening, doubling and reversal in the Astrophysical Journal of April (1906). 10 Bulletin Bureau of Standards, 2, No. 3, Dec. (1906). 11 Nutting, Astrophysical Journal, 28, 66 (1908). 12 Astrophysical Journal, 20, 135 (1904). 13 Ibid., 20, 284 (1904). cluded that a 'high E. M. F., rapidly changing, is a probable conditio sine qua non for the appearance of spark lines in arc spectra.' Both might better have expressed their results in terms of potential gradient." "The lowest gradients are obtained in heavy current arcs and Plücker tubes with wide capillary; in the former case the low gradient is due to the heavy current, in the latter to low gas pressure. Higher potential gradients are obtained in arcs with very small current, Plücker tubes with fine capillaries and sparks with small capacity and large inductance. The highest potential-gradients are found in sparks and other interrupted arcs, the gradient increasing with the amount of capacity in circuit and with the impressed voltage. Gradients vary from about 20 to 80 volts per cm. in ordinary arcs and tubes up to thousands of volts per cm. in condensed sparks.” * * "Inductance reduces the gradient down to a minimum, beyond which it is inoperative." *** "In the condensed spark without inductance, the front of the pilot discharge must have a potentialgradient not much below the dielectric strength of the intervening gas. The remainder of the discharge is probably at a very low gradient, approaching that of a direct-current arc. Hence such a spark gives both spark and arc lines. Inductance and resistance lower maximum gradients by smoothing out the current wave. The spectrum of a spark rendered dead beat by series resistance can scarcely be distinguished from that of a low direct-current arc." In 1909 Janicki 14 writes on the structure of spectrum lines, giving the results of a study made with the Lummer-Gehrcke plate, the source being an arc at low pressure (0.1 mm. or less) in a special form of apparatus having an anode of the desired metal. The three zinc lines in the blue are described as sharp and simple. They appeared at 0.3 amp., were good at 0.4 amp., and at more than 0.7 amp. were reversed in part. In certain calcium lines the change of position of their satellites with increase of current is noted, and attention called to an unsymmetrical broadening and reversal. Somewhat later reference is made to the work of Exner and Haschek on the displacement of spark lines. "They traced these displacements, directed mostly toward longer wave-lengths, to the different density of the metallic vapor. With good reason Eder and Valenta objected that these displacements were only apparent. *** They photographed arc and spark lines im 14 Annalen der Physik Band 29 (1909). mediately above one another with different exposure times. The long exposures seem to give a different center of intensity from the short, if a line is unsymmetrically broadened to one side; whereas on the other hand the real center remains clearly in the same position only in the case of sufficiently short exposures. The long and short exposures play the same rôle, however, as a greater or smaller density of metallic vapor; therefore the shifts observed by Exner and Haschek are to be considered only as apparent. Exner and Haschek then tried to maintain their theory by referring the cause of the shifts to changeable satellites, which cannot be resolved by a Rowland grating and might therefore produce a shift. They studied the arc lines of a series of elements by means of a 15 plate echelon and made the astonishing discovery that a satellite often appeared upon the red side of the line, especially when the arc flickered. With the plane parallel plates at my disposal, which are more efficient than a 15 plate echelon, I have been unable to verify the satellites which they reported." *** "It is possible that the satellites seen by Exner and Haschek with the flickering of the arc arose from impurities in the carbon and the metal. It is more probable, however, that they must be regarded as ghosts. Ca X 4527 is supposed to be simple, but with a satellite arising on the side of greater wave-length upon the flickering of the arc; whereas I found no satellite near this strong line. On the contrary, I observed a weak satellite of greater wave-length near Ca X 4586, while Exner and Haschek did not. Ca λ 5270 is supposedly a triplet, in which with weak current the middle line is the brightest; with strong current the two lines toward the red are the brightest. All my photographs show this very strong line to be single; furthermore, Cu X 5218 is supposed to have a red companion which grows more rapidly than the head-line as the current is increased; I always found this very strong line to be single. This very line seems to me proof that Exner and Haschek were deceived by ghosts in their echelon. For if the headline is not very strong, the ghost can scarcely be seen; if the main line becomes stronger, the ghost comes out more strongly; with further increase in intensity, the main line, however, seems to gain less rapidly than the ghost, since the eye (Exner and Haschek make visual observations only) cannot distinguish differences in great intensities so accurately as in the case of small ones. Nutting has also used the ordinary arc for creating spectrum lines and worked with an echelon of 30 plates, of 11⁄2 cm. thickness. The same remarks as above made are valid in case of the use of the carbon arc." Janicki reviews Nutting's results, characterizes them as extraor |