The Use ot a Simple
bated, not improved, by averaging. Figure 2A shows the noise free signal shape (#I) as well as the signal (#2) and the noise (#3) that were combined for averaging. In the sum, the signal is invisible. Figures 2B shows the signal acquired using 0.01-, 0.1-, and 1.0-s integration periods.
Arrangement for Teaching Atomic Theory in General chemistrycourses J. Casanova lnstihno de F i s h Facultad de Ciencias &icas y Matedtiticas Universidad Cat6iica de Vaiparaiso Casiila 4059, Vaiparaiso. Chiie M. Arellano, L. Lam, and H. G6mez InstiMo de Quimica Facultad de Ciencias Bdsicas y Matemdticas Universidad ~at6iicade Vaiparaiso Casiila 4059, Vaiparaiso. Chiie
I n introductory general chemistry courses, it is often necessary toexplain those experiments that support modern atomic theory. Among them the analysis of discharee tube eroeriments is imoortant for intrdhucine stuhents to the hvdroeen atom experiment that is easily performed during a lecture demonstration. The radiation rmitted by a hydrogen diarharre tubegws through theslit ofn homemade spectr&neter, the grating dispersing the radiation according to the well-known relationshio nA = d sin (8). . . Measurement of the radiation intensity inagivendirection is arcomplished hy use of a phototransistor plarrd at the focal point o f s lens located at the end of a tube, which can turn on an axis coincident with the planeafthe grating. The corresponding photocurrent is converted to a potential difference proportional to the light intensity. The angle 8 is measured with a ootentiometer that rotates with the tube eonraining the lens. Thus the potential difference is prop~rtionalU, the value of 8. Both signals arrive successively at the analogue gate of an ADC, connected to a Commodore 64 microcomputer. These two signals can he processed almost simultaneously, i.e., within 200 ms of each other. If the intensitv-anele measurement is reneated a t equal t i k r &rvals, a plot of hght'mtensity vu. wavelrngth can be dwplayed on thp mrcrocomputer screen na the tuhe containmg the phototransistor is rotated. This experiment can be performed easily, allowing students to gain a more detailed and actual picture of the hydrogen atom spectrum, thus bridging the gap between the p h y s i d phenomenon and the theoretical interpretation. ~~
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Time (s) F l p e 2. A. #1 k (he nobfrea signal otfset by 6 V. #2 is (he attenuated #1 (dhetby 5 V) mat was summed wilh 1Y3 merate me nolsv .. to . sianal - fa B. B, Recovered signals using differentcounter gate periods (periods in rsconds indicated above each CUNB). Each curve is offset by 0.1 V to improve separation.
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The benefits of a wider ,. eate are nmnouneed Even a 0.01-s period yields remarkable reecwery. Signilirant distortionsraused hy too wide a period are apparent for the 1-8 period. COUNTER in IBM Turbo Pascal 3 (suitable with minor changes for 4) is available from J. N. Demas for $5.00. Disks can be supplied in either 720K 3.5-in. or 360K 5.25in. formats. Alternatively, send the equivalent of a t Least $5 worth of quality diskettes (quad density preferred) plus the one to he used for copying. We can also send to your E-mail address. ~
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Acknowledgment We gratefully acknowledge the support of the Department of Chemistry and the National Science Foundation (Grant CHE 8600012).
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wohnjen, H.: ~essy.R. J. Chem E&. 1979, 56.153-156. 2Toney. J. H.: Demas. J. N. Rev. Sci. hm. 1982.53. 1082-1085.
Volume 66
Number 8
August 1989
A201
