J SOME DEMONSTRATIONS WITH THE OVERHEAD PROJECTOR C. W. KEENAN University of Tennessee, Knoxville
PIECES of special equipment used to carry out lecture demonstrations on the stage of an overhead projector have been presented in several papers in THIS JOURNAL.' This paper will describe the construction and use of several additional pieces which have proved useful in illustrating certain lecture topics. The stage of the projector is ten inches square. The basic platform for mounting models is a sheet of transparent plastic (Lucite or Plexiglas) which is ten inches square and one-fourth inch thick. On this rugged base are fixed those items which are to be permanent parts of the silhouette to be projected. Plastic parts are glued on with commercial cement or glacial acetic acid, heavy cardboard is held with cement, and wires and other metal parts are held with small screws or small nuts and bolts. Different parts of the assembly can be exaggerated or minimized in the silhouette thus making possible the presentation of a working model with the simplifying advantages of a line drawing. A typical assembly, designed to demonstrate the electrolysis of water, is shown in Figure 1. The outline of
tened to the platinum by the same screws that hold the platinum to the hase. The electrolysis of dilute sodium chloride with a trace of phenolphthalein added is usually shown. When the current is turned on, the solution immediately turns pink a t one electrode; soon bubbles of gas are seen. (Sometimes the electrodes must be touched with a stirring rod to dislodge the gas bubbles and make them more visible.) At this poiut the lecturer may write equations for the electrode reactions on the plastic base with black wax crayon and indicate the direction of electron flow in the wires. After use, the solution is flushed off with distilled water and the crayon notes are rubbed off with a soft cloth or tissue. I n Figure 2 a modified Danicll cell is shown. ( A model of the gravity type could be used as well.) Thr assembly simulates a porous cup, coutaiuing a deep blue copper sulfate solution and a copper electrode, dipping into a beaker containing zinc sulfate solution and a zinc elect,rode. The outer walls of the solution-containing plastic outline are one-half inch in height, while the out-
I L
Figure 2.
Modified Daniel1 Cell
the beaker is cut from a sheet of one-fourth inch transparent plastic. This outline is glued to the ten-inch square plastic hase thereby forming a shallow container which is watertight. Portions of the outline are painted black so as to appear as a line drawing of a beaker when projected on the screen. Two strips of thin platinum sheet serve as electrodes. Held to the plastic base by screws placed outside the solution area and crimped so as to pass over the upper rim of the container and hack down to the solution area, these thin platinum strips appear in silhouette as sturdy electrodes dipping into the solution. Wires leading to a d.-c. source are fas'For a bibliography see ALYEA,H. N., J. CHEM.EDUC., 33, A541 (1956).
Figure 3. Model of Apparatus Which Could Be Used to Demonstrate Charl-' Law
line of the lower part of the inner container is only onequarter inch high. The mires leading from the zinc and copper electrodes are connected via a movable copper strip switch to an electromagnet made from a piece of soft iron wound with wire. A compass needle mounted in a transparent rase serves as a sensitive detector of current flow. In use, the unmarked apparatus is placed on the projector, sufficient concentratsd CuSOI solution is added to the iuner container to almost fill it, then a solutiou of ZnSOn is added to the outer chamber until it overflows into the CuS04 across the top of the plastic barrier which simulates the porous cup. The compass is placed in the field of the magnet a t a point where it is subject JOURNAL OF CHEMICAL EDUCATION
