A LANTERN DEMONSTRATION
DIATOMIC MOLECULE CECIL L. WILSON The Queen's University of Belfast, Belfast, North Ireland
H
AZELHURST1 describes a model by which rotation, vibration, and rotation-vibration motions of diatomic molecules consisting of two dissimilar atoms can be represented. This model has a rather complicated system of pulley drives, and a t the J. CHEM.EDUC.,12,353 (1935).
same time it is such that it can only be demonstrated with advantage to a small number of people a t any one time. By using the basic principle of Hazelhurst's model, a lantern slide has been prepared which may be used to demonstrate vibration and rotation-vibration, and, by
a simple alteration, rotation motions. This simplified model loses the variety of speed ratios of rotation to vibration which can be obtained with the original model. On the other hand, it may be recommended strongly on the grounds that it shows clearly the principles of the motions, and the way in which rotation-vibration is derived; and that it can be demonstrated satisfactorily to any audience, no matter how large. Figure 1 illustrates the construction of the model. Two circular glass plates A1, A%,are mounted by brass wire springs B in rims C which are fitted with rack gears. D, E, and are themselves mounted and free to
rotate in the two faces of a rackwork lantern slide. The radius of the rack gear D on one rim is slightly smaller than that of the other, E. A pinion, F, is mounted on a handle G which slides through a short distance. When the handle is pulled out, the pinion engages with the rack of greater radius alone. When it is pushed in, the pinion rotates both racks in opposite directions. Friction between the two rims is reduced to a minimum, while the friction between the rim and the face of the slide is kept considerably larger. This is to prevent the loose plate from being turned by the moving one
when the handle is pulled out. Alternatively a catch may be provided to lock the loose plate in position when necessary (not shown in Figure 1). The two glass plates are coated with a suitable black paint. When this is dry, the lines on each face are cleared by means of a scriber or other pointed instrument. These diagrams (Figure 2) are similar to those used by Hazelhurst for his model. On one plate are two epicycloids plotted with opposite signs, while on the other are two straight radial lines placed so that when the two discs are in position one of the lines just covers the variation of the outer orbit, and the other covers the variation of the inner orbit. By placing the slide in a lantern, two spots of light are seen on the screen. It is usually preferable to
slowly this transition may be seen clearly, and by hastening the speed of rotation i t is possible to impart a good idea of the actual orbits of the two atoms. Pure rotation isobtained by clipping both discs in one mount. The depth of the mount allows this to be done. The model has, of course, a similar limitation to that of Hazelhurst, in that the molecule which is represented must be unsymmetrical, otherwise the orbits would cross. But several molecules with different amounts of unsymmetry may be represented by using several sets of glasses carrying different curves. The difference in size of the two atoms may be represented to some extent by making the lines which produce the spot of light representing the heavier atom thicker than the others. This must not, however, be emphasized too much, as otherwise the slide cannot be thrown suficiently out of focus without weakening unduly the spot of light representing the smaller atom. SUMMARY
A simple rackwork slide which represents rotation, vibration, and rotation-vibration in unsymmetrical diatomic molecules is described. With this, the motions and their relation to each other may be portrayed to large audiences. throw the slide slightly out of focus. On keeping the handle pulled out and rotating it, the spots of light vibrate about an unsymmetrically placed center. When the handle is pushed in, the vibratory motion passes into rotation-vibration. By turning the handle
ACKNOWLEEGMENT
The author wishes to thank the Workshop of the Physics Department of this University, where the model described was constructed. .
