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 Visible and Infra-Red Radiation 
 of Hydrogen 
 
 .11 rw 23 1926 
 
 DISSERTATION 
 
 SUBMITTED TO THE BOARD OF UNIVERSITY STUDIES OF THE JOHNS HOPKINS 
 
 UNIVERSITY IN CONFORMITY WITH THE REQUIREMENTS FOR THE 
 
 DEGREE OF DOCTOR OF PHILOSOPHY 
 
 BY 
 
 FREDERICK SUMMER BRACKETT 
 
 // 
 
 DMIY 
 
 
 BALTIMORE 
 1922 
 
Visible and Infra-Red Radiation 
 of Hydrogen 
 
 DISSERTATION 
 
 SUBMITTED TO THE BOARD OF UNIVERSITY STUDIES OF THE JOHNS HOPKINS 
 
 UNIVERSITY IN CONFORMITY WITH THE REQUIREMENTS FOR THE 
 
 DEGREE OF DOCTOR OF PHILOSOPHY 
 
 BALTIMORE 
 1922 
 
&C457 
 
VISIBLE AND INFRA-RED RADIATION 
 OF HYDROGEN 
 
 BY FREDERICK SUMNER BRACKETT 
 
 ABSTRACT 
 
 Infra-red spectrum of hydrogen to 4.5 /. The radiation from the central section 
 of a long discharge tube, such as the one used by Wood to extend the Balmer series 
 to the twentieth term, was analyzed by means of a rock-salt prism spectrometer, and 
 readings were taken with an extremely sensitive vacuum thermo-junction. Besides 
 some unidentified lines (Fig. i), five members of the Paschen series, three of them 
 new, were observed at the wave-lengths corresponding to the formula v = N(i/3* 
 i/m 2 ), where m = 4, 5, 6, 7, 8; also the first two members of a new series corresponding 
 to v = N(i/4* i/w 2 ), where #1 = 5, 6, were observed at 4.05 and 2.63^. According 
 to the Bohr theory these two series are due to electrons falling from outer orbits into 
 the third orbit and fourth orbit respectively. 
 
 Variation of the relative intensity of Balmer and Paschen series lines of hydrogen 
 with the current. For the long discharge tube used, the first Paschen line was found to 
 increase in intensity more rapidly than Ha, as the current was increased from one- 
 ninth to one-half ampere through a section 7 mm in diameter. 
 
 INTRODUCTION 
 
 Two lines due to hydrogen were observed by Paschen 1 at the 
 wave-lengths 18,751 A and 12,818 A. According to the Bohr 
 theory these are due to an electron falling into the third from the 
 fourth and fifth stable orbits in the hydrogen atom. 
 
 In view of the very low intensities of these spectral lines observed 
 by Paschen, the detection of a series due to an electron falling into 
 the fourth from orbits of greater quantum number has been con- 
 sidered improbable. In fact the lines mentioned above, forming 
 the first two membeis of the Paschen series, were so faint that in 
 order to observe them Paschen found it necessary to set the spectro- 
 graph in the correct position according to the wave-length predicted 
 by Ritz in consequence of his combination principle. 
 
 By using a very long hydrogen tube Professor Wood 2 found it 
 possible to abolish the secondary spectrum from the central posi- 
 tion of the tube, and photograph the lines of the Balmer series 
 down to the twentieth member. From the standpoint of the Bohr 
 theory, this increase in the intensity of the higher members of the 
 
 1 Annalen der Physik, 27, 537, 1908. 
 
 'Proceedings of the Royal Society, 97, 455, 1920; Philosophical Magazine, 42, 
 729, 1921. 
 
 603606 
 
RADIATION OF HYDROGEN 155 
 
 Balmer series might be attributed to increasing the probability of 
 transitions between the orbits of higher quantum numbers. That 
 being the case, we should expect an increase in the intensity of 
 lines of other series which are also due to transitions of the electron 
 between orbits of higher quantum numbers. 
 
 That this proved to be the case, and to a degree far greater 
 than anticipated, is the essential feature underlying the success 
 of the present investigation. 
 
 APPARATUS 
 
 The hydrogen tube of pyrex glass, used as the source, was about 
 one meter in length. A central portion 25 cm long was viewed 
 end on through an elbow in the tube. This portion of the tube 
 was 7 mm in diameter (inside dimension). This small-size tubing 
 was also used for a distance of about 10 cm on either side of the 
 portion viewed. The remainder of the tube was made of larger 
 diameter in order to reduce the resistance as much as possible. 
 The electrodes were of thin aluminum foil rolled into hollow 
 cylinders 8 cm long by 2 cm in diameter. 
 
 Hydrogen was introduced through a capillary sealed in near 
 one electrode. The tube was exhausted by a Gaede mercury 
 pump, communicating with the tube near the other electrode. 
 During observation the pump was run continuously, the pressure 
 depending upon the balance between the pump and the capillary 
 intake. 
 
 Alternating high potential was supplied by a 66oo-volt 5- 
 kilowatt transformer operating on a no- volt, 6o-cycle primary 
 circuit. The potential difference applied across the tube was 
 varied by introducing resistance into the primary. The highest 
 current maintained through the tube was a little more than half 
 an ampere. 
 
 The dark space about the electrodes was about 3 mm in length, 
 the positive column occupying almost the entire length of the tube. 
 Striation appeared only in the portions of the tube of larger diam- 
 eter, the central constricted portion being occupied by apparently 
 continuous luminosity. The secondary spectrum was noticeable 
 only near the electrodes. Throughout the greater part of the 
 
156 FREDERICK SUMNER BRACKETT 
 
 tube, the B aimer lines appeared with great brilliancy against a 
 practically black background when the tube was viewed through 
 a direct-vision prism. 
 
 The dispersion apparatus was a rock-salt monochromator of 
 the Wadsworth type. Concave mirrors of 6o-cm focal length were 
 used for both collimator and telescope. The clear aperture of the 
 prism was about 4 cm in diameter. Light only traversed the prism 
 once, the simple form of apparatus being chosen in order to avoid 
 scattered light. The slit widths were about i mm. 
 
 The detecting apparatus consisted of a single-junction vacuum 
 thermo-couple, of the type constructed and previously used by 
 Professor Pfund for measurement of stellar radiation. This was 
 connected in series with a d'Arsonval galvonometer of sensibility 
 about 5Xio~ 10 . 
 
 A scale-distance of three meters was used, giving altogether 
 an arrangement of extreme sensibility. 
 
 The light from the second slit was concentrated upon the black- 
 ened strip of the thermo-junction by means of a short-focus con- 
 cave mirror. Both mirror and thermo-junction were protected 
 by an asbestos housing. The leads to the galvanometer were 
 carried through a metal conduit, grounded to prevent electro- 
 static effects. The galvanometer case, posts, etc., were heavily 
 covered with cotton batting. The stability of this arrangement 
 proved very satisfactory, deflections being reliable to o.i mm. 
 
 RESULTS 
 
 Part I. Distribution of intensity. Figure i shows the distribu- 
 tion of intensity of radiation due to hydrogen in the infra-red 
 between wave-lengths 0.5 ju and 4.5 ju. 
 
 In this diagram intensities have been plotted as the ordinates 
 measured in millimeters of deflection of the galvanometer. It has 
 been shown that intensities are proportional to galvanometer 
 deflections for this thermo-couple for small deflections such as are 
 obtained here. The abscissae are simply micrometer turns of the 
 monochromator. The dispersion-curve based on five known 
 points of the monochromator is shown superposed. Its ordinates 
 
RADIATION OF HYDROGEN 
 
 are wave-lengths in microns indicated on the right. In order to 
 read the wave-length of any point on the intensity-curve it is 
 
 FIG. i 
 
 merely necessary to find the point of the dispersion-curve having 
 the same abscissa and read its ordinate at the right. 
 
FREDERICK SUMNER BRACKETT 
 
 The maximum occurring at 4.05=^.03 p and the second one at 
 2.63^.02 jii are the first two members of a new series predicted 
 from the Bohr theory by the formula 
 
 i/X= 109,677.7 (i/4 2 -i/w 2 ); m = $.6. 
 
 That is, they are attributed to an electron falling into the fourth 
 from the fifth and sixth stable orbits of the hydrogen atom accord- 
 ing to Bohr's model. 
 
 The largest maximum occurring at i .88 fj. is the first line of the 
 Paschen series. It will be noted that it has an intensity greater 
 than Ha (the last maximum on the left) in the ratio of about 4 
 to 3. The next four maxima are the next four members of the 
 Paschen series. 
 
 The identity of the succeeding maxima is uncertain. Other 
 lines are certainly present which do not belong to the Paschen 
 series. The second maximum is probably Pf . 
 
 These lines of the Paschen series are predicted from the Bohr 
 theory by the formula 
 
 i/X = 109,677.7 (i/3 2 -i/w 2 ); m = 4, 5, 6, 7, 8, (9), 10. 
 
 Line 
 
 Observed Wave- 
 Length 
 
 Calculated Wave- 
 Length 
 
 Pa.. 
 
 (l.88/i) 
 
 I 87? u 
 
 PS 
 
 (l 28 M) 
 
 1 . 282 fj. 
 
 P v 
 
 I 00=*= OI U 
 
 I OQ4 u 
 
 P5 
 
 I .OI=*= .OI fj. 
 
 1 .005 fj. 
 
 Pe 
 
 O.QS^ .OI U 
 
 O.O<? u 
 
 
 
 
 The dispersion-curve from which the wave-lengths have been 
 determined is based upon five known points: 4.4 /j. and 2.7 n of 
 the Bunsen flame (shown by dotted lines scale reduced 100 times) 
 and the three known hydrogen lines Pa, P/3, and Ha. The values 
 thus obtained should be quite satisfactory for purposes of identi- 
 fication, but of course have no great value from the standpoint of 
 accurate wave-length measurement. 
 
 In view of the great intensity of the Paschen lines it is not 
 surprising that many lines were readily observed which under 
 ordinary conditions of excitation would not be detected. The 
 
RADIATION OF HYDROGEN 
 
 159 
 
 readings shown on Figure i were taken in sequence, without setting 
 upon known wave-lengths. The identity of the maxima was not 
 realized until the dispersion-curve was later plotted. 
 
 Part II. Variation of intensity with current. As it was noticed 
 that the appearance of the tube changed decidedly with change 
 in current, a set of intensity readings was taken on the first lines 
 of three series the Balmer, the Paschen, and the new series, when 
 the current through the tube was varied from one-sixth of an 
 ampere to over one-half an ampere. The curves obtained are 
 shown in Figure 2. 
 
 10 If jt Ante 
 
 FIG. 2 
 
 The ordinates are intensities in millimeter deflections of the 
 galvanometer, and the abscissae are current strengths through 
 the primary of the transformer. The current through the tube 
 is found with sufficient accuracy by dividing by 60. Readings on 
 a of the new series are not shown for currents greater than 20 
 amperes primary, one-third ampere secondary, as radiation from 
 the glass tube itself rendered them uncertain. No such effect was 
 observed for lower currents or wave-lengths less than 3.5 p. As a 
 further precaution the tube was run only during the time of actual 
 reading. 
 
i6o FREDERICK SUMNER BRACKETT 
 
 From this diagram, Figure 2, it will be noticed that although all 
 three lines increase in intensity with increasing current, Pa increases 
 by far the most rapidly. Further readings (dotted portion of the 
 curve) showed that at currents less than 7 amperes primary (about 
 one-ninth ampere secondary) Ha was more intense than Pa. 
 
 At a current of 23 amperes through the primary we have the 
 ratio of the intensity of Pa to Ha a little more than 4 to 3. Accord- 
 ing to the Bohr theory the energy lost by an electron undergoing 
 transition from the fourth to the third orbit of the hydrogen atom 
 will be 
 
 W P a=(i/3 2 -i/4') Nh = 
 
 while that lost in transition from the third to the second orbit 
 would be 
 
 W H a=(i/2 2 -i/ 3 2 ) Nh= 
 
 So the energy given out by an electron in transition from the 
 fourth to the third orbit is only seven-twentieths of that given out 
 by an electron falling from the third to the second orbit. Hence, 
 in order to have Pa more intense than Ha in the ratio 4:3, there must 
 be more atoms radiating Pa than Ha in the ratio of 
 
 In view of the greater stability of the orbits of smaller quantum 
 number and taking into account the principle of selection, we see 
 that this is incompatible with what would be expected if radiation 
 were resulting only from recombination of the atom and electron 
 after ionization. We must therefore conclude that to a large extent 
 radiation in such a long hydrogen tube arises from inelastic collision 
 without ionization. In such a case we may have an abnormal 
 concentration of energy in certain wave-lengths which would not 
 be the case for recombination after ionization, inasmuch as the 
 probability of an electron stopping in an orbit of high quantum 
 number is much greater, where it is simply ejected to that orbit or 
 one of slightly greater quantum number, than in the case where it 
 
RADIATION OF HYDROGEN 161 
 
 is returning from oo . This no doubt explains to a large extent the 
 peculiar characteristics of the "long hydrogen tube," both in regard 
 to the unusual intensity of the infra-red lines and also the higher 
 members of the B aimer series. 
 
 All atoms which contribute to the radiation of the first Paschen 
 line are left necessarily with the electron in the third orbit, whence 
 it must proceed either to the second or the first orbit unless it is 
 ejected to some orbit of greater quantum number by inelastic 
 collision. Since considerably less than one-fourth of these atoms 
 contribute to the radiation of Ha, we must conclude either that 
 the second member of the Lyman series is radiated with great 
 intensity, or that there must be multiple collisions to a large extent 
 in such a tube. 
 
 It should be noticed that the higher members of the Paschen 
 series occur in a region readily studied by photographic methods, 
 plates hypersensitized by means of dicyanin being sensitive to i.o p. 
 This work will be carried out in the near future. 
 
 CONCLUSION 
 
 1. The first two lines of a new series have been observed at 
 wave-lengths 4.05=*= .03 ju and 2.63=1= .02 /*, due, according to Bohr's 
 theory, to an electron falling into the fourth from the fifth and sixth 
 rings of the hydrogen atom. 
 
 2. Three and probably four additional members of the Paschen 
 series have been observed. 
 
 3. The first Paschen line is more intense than Ha in the ratio 
 4:3 under the conditions prevailing in the long tube for great current 
 densities. 
 
 In this work I have been much indebted to Dr. Pfund, who 
 proposed the problem and constructed the thermo-junction, the 
 remarkable sensibility of which rendered the work possible. 
 
 It gives me great pleasure to express my appreciation to him, 
 to Professor Wood for his interest and many suggestions, and to 
 Dr. Ames for his generous support. 
 
 JOHNS HOPKINS UNIVERSITY 
 June 1922 
 
BIOGRAPHICAL NOTE 
 
 Frederick Summer Brackett, son of Frank Parkhurst and Lucretia 
 (Burdick) Brackett, was born August i, 1896, in Claremont, California. His 
 preliminary education was obtained in the Claremont public schools. He 
 graduated from the high school in 1910, entering Pomona College the following 
 fall, where he pursued the general course, majoring in physics and mathe- 
 matics. He was elected to Phi Beta Kappa in the spring of his Junior year. 
 He graduated cum laude in February, 1918, receiving the B.A. degree in absentia. 
 
 He was laboratory assistant in the optical section of the Bureau of Standards 
 for the year 1918-19, at the same time pursuing courses in advanced physics 
 and mathematics given by Professor Ames. He was a solar observer on the 
 staff of Mt. Wilson Solar Observatory during 1919-20, the published results of 
 his research being An Examination of the Infra-Red Spectrum of the Sun A. 8goo 
 to X 9900 A. 
 
 He was appointed instructor in physics at the Johns Hopkins University 
 in the- fall of 1920. While there during the following two years he pursued 
 graduate courses in physics, mathematical physics, and dynamical geology under 
 Professors Ames, Murnaghan, and Reid. 
 
 He was an assistant physicist at the Bureau of Standards during the summer 
 of 1920, being retained through the winter in a consulting capacity. 
 

 
 OCT 
 
 OEC -6195* 
 
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