Gregory V. Nelson Drew University Madison, New Jersey 07940
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stant TV Replay in the Lecture Hall
recent article describing modern teaching aids mentioned a television recording cart used to videotape experiments in the research laboratory and bring them into the lecture hall [BARNARD, W. R., LAGOWSKY, J. J., AND O'CONNOR, R., J. CHEM.EDUC., 45, 63 (1968)]. A demonstration hench incorporating a camera for live demonstrations was also mentioned. At Drew University we are equipped with a rearprojection screen and a television projector which can he connected to a modified version of the television cart. When closed and locked, the cart is a portable unit of the lecture hench. When opened and connected, the unit in the lecture hall is used for live demonstrations, or disconnected from the projector, it may he used to videotape experiments in the researeh laboratories. Our experience using the television recording cart has been remarkably successful. The projected image enables the students to see very small details. (A penny enlarged to fill the screen reveals Lincoln's statue in the center of the Lincoln memorial engraved on the tail's side.) A particularly useful technique in the lecture demonstration is "instant replay." As an example, consider a demonstration of supersaturation. The saturated solution of sodium thiosulfate is heated in a small test tube with excess solid before the TV camera. After the solid has dissolved, the tuhe and its contents are cooled in a beaker of ice water. This
620 / Journal of Chemical Education
first part of the demonstration may he watched either on the big screen or directly. Whiie the sample is cooling, the departure from equilibrium is explained. Then just before adding a seed crystal to it, the cooled tuhe is mounted on a ring stand in front of the camera, projecting an image a t least half the width of the screen. The videorecorder is started, recording what is projected, and a seed crystal is added to the supersaturated solution. The initial explanation deals only with what has been performed. The student usually sees in addition to the growing seed crystal, a "vapor trail" of small crystallites forming behind the crystal as i t falls to the bottom of the tuhe. These small particles grow quickly to fill the tube with crystals. After viewing the crystals forming, the student,^ may he asked to explain what they saw. Then, by "instant replay," the critical part of the demonstration may he repeated without further preparation, showing the identical details. The teacher then gives a summary of the description in terms of establishing equilibrium. The demonstration combines the honesty of a live demonstration with an efficient means of reinforcement by repetition. The large-screen presentation is drsr matic enough that the students have suggested other demonstrations they would like to see and have helped produce them. This work was supported by the National Science Foundation COSIP Grant GY-3788.
