TEACHING WEIGHING TECHNIC WITH THE AID OF A MOTION PICTURE FILM' SEBASTIAN A. DURBAN Wright City Junior College, Chicago, Illinois
THE usual method of instruction employed to teach analytical balance technic in the first course in quantitative chemical analysis is to divide the class of students into small groups of four or five who gather around the balance to watch the instructor demonstrate the accepted methods of handling the balance controls and the other necessary operations. This procedure must be repeated several times until the entire class has seen the demonstration. Discussion and illustration of the method of calculating the final weight of the sample is usually reserved until a later time when the entire class is assembled. In using this time-honored and timeconsuming method the student does not have adequate opportunity to see the detailed technic of weighing and he does not have presented to him the whole process of weighing and calculation of the final weight of the sample. With the triple objectives of (1) shortening the time required for students adequately to absorb weighing fundamentals, (2) increasing the effectivenessof teaching balance technic, and (3) making the study of quantitative analysis more interesting for the student, we have produced a motion picture film incorporating dl1 essential details of the weighing procedure. The film has already been used and tested with two of our regular classes in quantitative analysis and has proved to be far superior to the usual method of instruction. The film, contained on two 400-foot reels, is of the silent 16-mm. type, 531 feet in length. An unusual feature is the provision for brief pauses to allow for calculations to be made on the board by the instructor. This is accomplished by the use of "STOP" titles spliced into the reel a t appropriate points. The operator is instructed to watch for these "STOP" titles, and to stop the projector for a few minutes a t each one of them so that the lights may be turned on in the room and the instructor may make a calculation on the blackboard: For instance, immediately after a "rest point," "sensitivity," or "final weight" has been demonstrated on the film, the instructor records the results in tabular form on the blackboard and, a t the proper place and Presented before the Division of Chemical Education a t the 102nd meeting of the A. C. S., Atlantic City, New Jersey, September 11. 1941.
time, demonstrates on the blackboard how the final weights are calculated. In this manner the student is given an integrated demonstration of weighing operations and the methods of calculation of the final weights by the sensitivity method. The technic of using this type of film as a teaching aid is of course most important. We have found the following procedure to be most effective and believe that it could be successfully employed with either small or large classes: First Meeting of lh,e Class.-This consists of a discussion of the theory of weighing (rest points, sensitivity, etc.) and analytical balance construction and characteristics, and also an examination of sets of analytical weights. Just prior t o this period the c l a s has been assigned the chapter in the text dealing with the subject of weighing. The method of weighing by sensitivity is thoroughly discussed. Second Meeting of the Class-At this meeting a lecture demonstration, employing the film described, is given by the instructor. As noted, the projection of the film is stopped a t appropriate intervals and blackboard calculations of rest points, sensitivities, and final weights are made by the instructor. At these "stop" intervals, opportunity is also given for questions from the class. The film progressively illustrates the following technics: preparation of the balance before weighing; handling of the balance controls; determination of rest points and sensitivities; actual weighing of (1)a simple metal object. (2) tare of watch crystals, and (3) a powdered ore sample. When the showing of the film has been completed the class has seen the weighing of three separate objects and the complete calculation of their final weights. During the entire time that the film is being shown the instructor stands near the screen with a pointer and calls attention t o those technics that require emphasis.
Since we have found this film to be entirely satisfactory as a substitute for the traditional method of instruction i t may be that others would like to use it. Correspondence with the author is invited. The author wishes to acknowledge and to thank the following for their valuable advice and assistance in this work: Dr. Nicholas D. Cheronis, Head of the Department of Physical Sciences; Dr. (Mrs.) B. B. Freud, Instructor in Quantitative Analysis; Dr. Willis C. Pierce, Professor of Analytical Chemistry, University of Chicago; Mr. Clarence Fawcett, Instructor in Qualitative Analysis; and Mr. Joseph Lubinski, student assistant.
