R. A. Hermenr Millikin University Decatur, Illinois
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Teaching Aid for Three-Component Systems
One problem encountered in the instruction of the phase rule of three-component systems is drawing the equilateral triangle precisely and rapidly. As a rule the divisions within the triangle are not uniform in a hand-drawn sketch. Time in lecture does not permit careful measurement of several of these diagrams each period. For those who have tried to evaluate a point on a hand-sketched, three-component diagram, it is evident that the apparent percent composition A B C does not equal 100. Adding tielines to the graph complicates the blackboard drawing even further. An inexpensive and versatilc teaching aid for threecomponent systenls can be easily constructed aa follows: A precise triangnlar graph is drawn on a sheet of white paper with coordinates properly labeled. An industrial printing firm can photograph this and print it on paper about 20 inches square, (a cost of $5.) The backing is light weight sheet metal on 'I4 in. plywood. The photographic paper can be attached to the metal by photographic dry-mount tissue. The tie lines and movable points on the graph are made of in. plastic tape with a magnetic backing. The t,ape also has an
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adhesive surface on the side opposite t,he magnetic backing.' Removable notations (such as compound names) can be H. 0 placed on the adhesive side which then can be attached to the graph by the magnetic side. Predrawn phase diagrams such as immiscible liquids and two solids and a liquid A can belightly drawn -in different colors. X These can be used to set up instructional illustra.tions rapidly during class with the removable magnetic tape as tie-lines and phase-boundary lines. With the aid of a cork borer, one can bore 8-mm circles from the magnetic tape to use for points a t the end of tie-lines.
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' Edmund Scientific Co., Barrington, N. J.
