In the Classroom
edited by
Overhead Projector Demonstrations
Doris K. Kolb Bradley University Peoria, IL 61625
An Apparatus for Temperature Displays George Papageorgiou* and John Xenos Laboratory of Chemistry, Department of Primary Education, Democritus University of Thrace, Alexandroupolis 681 00, Greece; *
[email protected] In the literature, there is a lot of interest in lecture demonstrations involving measurements of temperature (1–6 ). Some of the articles refer to constructions that provide the possibility to display these measurements using an overhead projector (4–6 ). In this context, we describe a low-cost apparatus that is easy to construct and allows the temperature of thermal phenomena to be displayed using an overhead projector. Figure 1 shows the apparatus. There are three main components: (i) an ammeter, the screen of which can display the temperature values when placed on the overhead projector, (ii) a glass tube with the temperature sensor, and (iii) a box, which contains a battery, a variable resistance, and an on–off switch. Some of the advantages of the apparatus are as follows: 1. Construction requires only simple and inexpensive materials (the total cost is less than $20 in Greece) that are available in electronic shops. 2. The temperature sensor presents a linear behavior (between indications of the screen and temperature) in the range 0–100 °C. 3. The apparatus can be calibrated easily.
The Construction First, the upper half of the back cover of the ammeter must be cut. Because in many ammeters the inside screen is not transparent, the following procedure should be followed. 1. Remove the front (transparent) cover of the ammeter. 2. Unscrew the inside screen of the ammeter. 3. Unscrew the basic unit of the ammeter and remove it from the back cover. 4. Cut out the upper half of the back cover of the ammeter. 5. Screw the basic unit of the ammeter to the lower back cover. 6. Replace the front cover. 7. Copy the numerical scale of the screen of the ammeter onto a transparent sheet (each µ A corresponds to a Celsius degree) and attach it to the front cover.
4. The apparatus provides analog displays. We believe that the rate of the needle’s movement on the analog screen provides the possibility to monitor the rate of the phenomena (especially the fast ones) more easily than the digital display. 5. The apparatus can be used for measurements in strong acid or base solutions. 6. The apparatus can be used in the traditional high school classroom or any other lecture hall.
Materials An ammeter 100 µA (internal resistant 4 kΩ); A temperature sensor chip, LM35; A glass tube about 20 cm long and 0.5 cm in diameter; Heat-transfer liquid silicone; A 9-V dc battery; A switch (on–off ); A 10-kΩ variable resistance; A rubber stopper suitable for the glass tube; Wires for connecting the ammeter and the sensor with the battery, as shown in Figure 1. Figure 1. The apparatus. 1: 100-µA ammeter. 2: Temperaturesensor chip. 3: Glass tube. 4: Liquid silicone. 5: Rubber stopper. 6: 9-V battery. 7: On–off switch. 8: 10 - kΩ variable resistance.
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Journal of Chemical Education • Vol. 76 No. 8 August 1999 • JChemEd.chem.wisc.edu
In the Classroom
Similar procedures have been followed for the modification of ammeters in analogous constructions for measurements of conductivity, electromotive force, and voltage (7, 8). A tube with temperature sensor is constructed as follows: 1. Connect three wires in the three tips of the sensor chip. 2. Cut a piece about 20 cm long from a glass tube with a diameter of about 0.5 cm. 3. Pour liquid silicone into the tube from one tip, so it fills to a height of at least 1 cm. 4. Close this tip of the tube with a rubber stopper. 5. Put the sensor chip (with the wires) into the tube and immerse it into the silicone. 6. Close the open tip of the tube with a tape.
The circuit is then connected, as shown in Figure 1.
To calibrate the instrument, a beaker with water (or other liquid) is heated for a few minutes. A simple thermometer and the sensor’s tube are immersed in the water. The variable resistance is regulated so that the needle of the ammeter shows the same value as the thermometer. Literature Cited 1. 2. 3. 4. 5.
Dutton, F. B. J. Chem. Educ. 1955, 32, 478–480. Estok, G. K. J. Chem. Educ. 1960, 37, 303. Hur, C.; Solomon, S.; Wetzel, C. J. Chem. Educ. 1998, 75, 51–52. Alyea, H. N. J. Chem. Educ. 1962, 39, A381–A382. Barnett, B. E.; Pollock, J. A.; Battino, R. J. Chem. Educ. 1986, 63, 460. 6. DuPré, D. B.; Just, W. E., J. Chem. Educ. 1994, 71, 691. 7. Alyea, H. N. J. Chem. Educ. 1962, 39, A300. 8. Alyea, H. N. J. Chem. Educ. 1966, 43, A539, A586.
JChemEd.chem.wisc.edu • Vol. 76 No. 8 August 1999 • Journal of Chemical Education
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