Educational screen & audio-visual guide (c1956-1971])

The Filmstrip Projector For Science Experiments hij Ralph S. Vrona, Teacher of Science New Lincoln School, New York 26, New York J.T IS tempting to consider "other" uses for the filmstrip projector, an intense Hght source, which can magnify. What else besides 2x2 transparencies and filmstrips can be projected? How intense can the light beam be kept? In most projectors, experiments are limited by the fact that materials must be small and flat to fit into the instrument for focusing. Thicker objects can be projected however, if the focusing lens is removed from its barrel and placed in front of the instrument while the object to be focused is held behind the lens, as shown in Figure 1. Here are several ways we use projector for class-viewing objects which ordinarily can only be seen individually. J. Dental X-rat/s— Dental x-rays fit neatly into 35 mm slide holders. If the filmstrip projector has a slide attachment, these x-rays can be shown to the class, and their quality is good enough for observation of roots, nerves, pulp, dentine, fillings, as well as the original use in detecting dental troubles. Dentists usually have old x-rays which they will gladly donate and it is a good idea to get as much information about the particular set as possible. If the projector has no slide attachment, it is still possible to project these x-rays by removing the lens from the barrel, placing the x-ray to be viewed at the open end of the barrel and focusing the lens while held in the hand or (for steadiness) while held by a clamp and ring stand. (As in Figure 1. ) These x-rays can be printed onto directpositive film, or onto fine-grain positive film for ordinary viewing, if the facilities of a dark-room are available. When the x-ray is placed on the film and exposed to light, other portions of the film will also be exposed. Therefore it is advisable to mask off imused parts of the film with dark paper. In this way several x-rays can be put on a filmstrip. Fine-grain positive film will reverse the x-ray. 2. A vibrating filament— Aitei the filament of a transparent electric light bulb burns out, there is often a V4 inch or Vz inch stub of the tungsten remaining on one of the suports. When the bulb is rapped gently on a table top, this section of the filament vibrates, and since there is a high vacuum in the bulb, vibration will continue for some time. This vibration can be projected, as an example of harmonic motion, using the technique pictured in figure 1. There is ample time, before the vibrations die down, to "stop" them with a stroboscope such as the one used in the PSSC Physics course. There is an advantage here in that the projected image is large enough for study by the whole group. The bulb should be held securely (by a clamp and ring stand) or the vibrating filament will go in and out of focus. Long narrow bulbs used to light aquaria are most easily projected. If the bulb is clamped so that it is in contact with the projector barrel, it will pick up sufficient vibration from the projector motor to keep the filament stub vibrating continually. 3. Projecting tuning fork vibrations— An interesting project in General Science or Physics is to smoke a piece of glass and move a tuning fork across it rapidly and gently. The timing fork leaves a wavy line where it rubbed the smoke from the glass. (A small nail should be attached at right angles to one of the arms of the fork with a rubber band and the point of the nail allowed to scratch on the smoked plate). The vibrations of tuning fork on smoked glass are small in amplitude, not easily seen with the unaided eye. If the smoked glass is held in front of the projector barrel using the technique in figure 1, these vibrations appear gigantic when projected. Differences in frequency can be compared by making streaks with several tuning forks close together on the smoked plate. 4. Testing pin-hole carneras— Pin-hole cameras are easily constructed from small milk cartons. A pin hole is punched in the middle of the bottom of the carton; the top is removed and replaced with wax paper. Usually however, there is so much light in the room that it is hard to detect an image on the waxed jiaper. If the room is darkened and a filmstrip projector is used to throw a picture (preferably in color and with good contrast ) on the wall, its image can easily be detected with the camera. In order to make the picture as bright as possible, the distance of projection should be limited. Incidentally, if the pin-hole is too small there won't be sufficient light to form a detectable image on the waxed paper. The inside of the milk carton should be painted black with tempera paint, mixed with a little soap flakes to make it stick to the waxed surface of the carton. My students' first observation is always that the image they see on the waxed paper is upside down. 5. Convection ci/rrcn/s— Although we do not often see hot air rising, indirect methods can be used to determine convection currents. If we project the beam of a filmstrip projector onto a wall, and place a lighted candle in the beam about two feet from the wall, convection currents can be seen rising like wavy shadows, from the candle. It is interesting to raise the beam of the projector to see how far above the candle 658 Educational Screen and Audiovisual Guide — November, 1962