A Simple and Permanent Tyndall Cone Apparatus HAROLD J. ABRAHAMS and ABRAHAM DUBNER Central High School, Philadelphia, Pennsylvania
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HE APPEARANCE of a cone when a beam of light IS passed through matter in the colloidal state was noticed by Faraday. It was also studied by John Tyndall, after whom the phenomenon was named the
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apparatus. It would thus seem to be highly desirable to set up an apparatus which would once andfor all be rigidly fixed so as to be ready for use a t a moment's notice. Suchacontrivance wouldbefool-proof, economical of time, independent of weather conditions, andcompact. ~ x ~ e r i m e n t a t i owith n these requirements in hind has resulted in the apparatus herein illustrated. (See
1.-SIMPLEA N D PERMANENT TYNIIALI. CONEAPPARATUS
"Tyndall cone effect." This phenomenon is a beautiful one and generally arouses much interest in the student of chemistry. Although it should be easy to demonstrate, a large number of teachers omit to do so for lack of a good light source or because it is not always easy to set up a sharply focusing system in a short time. Having made the necessary adjustment of distances between light source, lens, and specimen of colloidal matter, there is no carry-over for the next school term so that the demonstrator must again waste time and tax his patience while he stumbles once more, by trial and error, upon the correct spacing of the parts of the
a = glass top; b = lens; c = lens-support (wooden);Td = wooden box or case; e = 6.3-volt radio pilot lamp; f = "C. H." toggle switch; g = cord: h = 6-volt, bell-ringing transformer; r = light rays.
Figure 1.) The f 3.0 lens (see b, Figure 2), one inch diameter or +l5 diopters, was taken from an old, very inexpensive moving-picture projector. The bell-ringing transformer, h, is of 6-volt rating, to which a 6.3volt radio pilot lamp, e, is connected. A small toggle switch (Cutler-Hammer), f, is placed into the circuit for convenience, although i t is hardly necessary and may easily be omitted. To determine the exact position of all of the parts before Iixing them rigidly, the lens first is supported by a ring stand, the ring of which is very small. The lamp and transformer are then placed a t the base of the stand and a beaker containing matter in the colloidal state is placed upon another ring above the lens. The contents of the beaker may be very dilute starch paste or water containing a few drops of India ink. Alittle cylinder of paper is slipped over the lamp to reduce its glare upon the experimenter's eyes. The room is now made totally dark. The positions of lens and beaker are varied with respect to the lamp until the best cone is obtained. Measurements of these distances between lamp, lens, and beaker are now made and recorded. More brilliant cones may be obtained by using larger lamps, but the use of these requires higher current transformers and consequent provision for ventilation to take care of the heat generated. No such provision is necessary with the parts used in our apparatus. A wooden box, d, with window-glass top, a , is now built from the measurements ascertained by experimenting with the ring-stand apparatus described above and the parts perma&tly encased as shown in Figure 2. The lens may be emplaced over the perforation of either a wooden or a metal support. I t is wise to make the box of such design that one wall may be easily removed in order to facilitate inspection and repair of the apparatus when necessary. The apparatus may now be painted. The use of this apparatus for demonstrating the Tyndall cone effect is very simple and very satisfactory. It is only necessary to take it and a glass cylinder or beaker containing colloidal matter into a blacked-out room. With the cord plugged in, the cylinder or beaker is placed upon the glass top of the box and a beautiful cone results. Figure 3 is a photograph of results ob-
tained with starch. India ink and gamboge may be used with equal success. These materials are used in very great dilution. The starch is prepared by adding a few drops of a thin starch paste to about 100 cc. of water and stirring. Either more water or starch is then added as dictated by the sharpness of the cone obtained. The procedure for preparing the India ink is similar to that for starch, a few very small drops being used per 100 cc. In the case of colloidal gamboge only a small pinch of the material is required. The entire apparatus can be built a t the cost of less than two dollars. if second-hand parts are not available.
