A. Companion and K. Schug Illinois Institute o f Technology Chicago
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The Spectrum of Atomic Hydrogen A mass scale freshman laboratory experiment
Hollenberg's recent suggestion1 on measurement of the Balmer lines of atomic hydrogen with a Beckman DU Spectrophotometer prompts us to describe our similar experiences in large freshman classes with the inexpensive student-built spectroscope described previou~ly.~The Model 2 spectroscope, when calibrated with a low intensity mercury Pluecker tube, despite the error in the assumption of linear calibration,* is capable of two to three figure accuracy in measurement of the wavelengths of the three lowest energy Balmer lines in the visible emanation of a low pressure H, Pluecker tube. For example, two typical student results are 6530,4890,4330 A and 6550,4850,4350 A, as compared to literature values 6563,4861,4320 A. Two approaches, both originated by students, are recommended for evaluating the Rydberg constant R without prior knowledge of the quantum numbers involved in the electronic transitions. In the first, students derive from the Rydberg relation: l / A = R[l/aa - l/ne]
the following equality involving their measured wavelengths X i : Al[l/az - 1/nt2] = Xz[l/aa - l/n?I = As[l/a2 - I/ns21
and, under the assumption of constant lower quantum number a, attempt to find pairs of integers (with ni> a)
'HOLLENBERQ, J. L., J. CHEM.EDUC.,43,216 (1966). W. W. AND COMPANION, A. L., J. 'EDWARDS, R. K., BRANDT, CHEM.EDUC.,39, 47 (1962). aHnnnrs, S. P., J. CHEM. EDUC.,39, 319 (1962). 'Central Scientific (CENCO), 1700 Irving Park Road, Chicago, Illinois 60613.
best satisfying this equation. The vaues 2, 3, 4, 5 are very rapidly deduced. In a second approach, three possible values of the Rydberg constant (one for each measured wavelength) were calculated for assorted pairs of integers a and n, a table of these was constructed as shown below, and the table was searched for consistent values of R. Over a range of integers from 1 to 7, no results were as conclusive as those illustrated. Table of Possible Rydberg Constants (in
cm-I)
Mercury and hydrogen Pluecker tubes and the associated power supply may be purchased for less than $50.4 Although the tubes are relatively inexpensive, reduction of the applied voltage with a Powerstat to 40 volts generally triples the lifetime of a new tube. An increase in voltage is necessary as the tube ages. One set of apparatus adequately services a t least 25 students during a 2-hr work period. Since the power supply delivers DC current (at times ca. 5000 volts), only instructors have been permitted to change the source tubes. Students are required to wear safety glasses at all times as a safety precaution against ultraviolet radiation. The experimental results and analytical procedures described here are summarized from laboratory reports of I I T students John Phelps, Leo Kramer and Craig Waryjas.
Volume 43, Number 1 1 , November 1966
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