|
Make sure components are mounted with 1/8 inch of clear space on both ends of
the board and that the components do not extend beyond the edges of the board. Then drill a 3/16-inch hole near the top of the enclosure for the piezo element. Scratch the inside
of the enclosure with sandpaper, align the hole in the piezo element with the hole in the enclosure, and cement together. Drill mounting holes and secure the on/off switch with the nuts provided. Banana jacks are mounted to the removable front panel of the enclosure by drilling two holes in the panel and securing with the hardware provided with the jacks. Copper wire electrodes (#12) can be soldered to the banana plugs and inserted into the banana jacks. The wires from the circuit board should be soldered to the lug provided with each jack.
The circuit board can be constructed on perf-board as shown in Figure 4. Insert the IC socket and two capacitors into the perf-board. Build the circuit board using an IC socket with the IC removed from the socket. This will prevent static electricity from destroying the device during construction. Note that pins 5 and 9 are connected to pin 7 of the IC. It is standard procedure to connect all unused inputs of a CMOS device to ground, as it reduces
current drain and increases battery life. One wire from the on/off switch should be connected to the red wire from the battery clip, and the other wire to pin 14 of the IC. Then connect the black wire from the battery clip to pin 7 of the IC. Connect the battery clip, the on/off switch, the wires going to the probes, and finally, the wires to the piezo element
to the IC unit as shown. After all connections have been made, plug the IC into its socket. Double check your wiring to be sure there are no unwanted solder bridges and that all connections are properly made. Slide and mount the perf-board and probe holder into the enclosure between the piezo element and the on/off switch as shown in Figure 3. It
should stay in place securely, but you can use a drop or two of glue to hold it in place. Secure the case with the two screws provided, insert a fresh 9-V alkaline battery, replace the battery cover, and you are now ready to perform conductivity experiments.
Experiments
We have discovered a novel experiment in which the relative resistance of various widths of pencil lines drawn on paper can be determined (see Fig. 1). Have students draw two filled-in circles with a pencil on a sheet of paper such that each probe can be pressed onto the center of each circle. Each circle should be approximately 1/8 inch in
diameter. Next have students draw a line connecting the two circles. At this time, a very slow (motorboating) sound may be heard. Now have students gradually increase the
thickness of the line connecting the two circles. While thickening
the line, an obvious increase in the frequency of the tone
will be heard. Students may also want to draw several
separate lines connecting the two circles following different
paths. These too should have the same effect as making one
line and then making it thicker. As the width of the line is
increased, the resistance between the two circles
decreases. This is because the effective resistance of a parallel
electrical circuit decreases as more resistance is connected in
parallel. Making the lines wider adds more paths for
electrons to flow between the two points. Various fluids can also
be tested using this device. Try, for example, distilled
water, tap water, vinegar, dilute hydrochloric acid, and salt
water. Different resistances indicate different concentrations
of electric currentcarrying ions in the liquid. A
particularly interesting experiment that is well demonstrated using
the audioconductivity apparatus is to test distilled water
and note the pitch of the tone heard. Upon adding ordinary
table salt to the water (adding sodium and chloride ion,
Na+ and Cl{, respectively), an increase in pitch of tone is readily
noticeable. A similar experiment involves the dilution of a
salt water solution with distilled water. A decrease in the
pitch of the tone should be noticed.
Other Ideas
Test an orange. First test the peal, then pierce the
orange with the probes. Any difference in tone? How does
the conductivity of an orange compare with that of a
lemon, apple, and potato? Remember, for accurate results, it is
essential that the probes be cleaned with distilled water
between tests to avoid contamination.
Conclusion
Other classical experiments reported in the
literature can be performed using this audio conductivity device.
The apparatus is inexpensive and easy to build and provides
one of the few audio demonstrations available (besides an
explosion) for the science classroom. Because it is audible,
we have found it to be a fun activity that focuses students'
attention and provides a strong foundation for
understanding the concepts of conductivity.
Acknowledgment
We would like to acknowledge the Research
Corporation Cottrell Grant Program for financial support of
this project.
Notes
1. An affordable version of a LED conductivity tester is
available from Science Kit and Boreal Laboratories, 777 East
Park Drive, Tonawanda, NY 14150.
2. Step-by-step directions for the construction of this
conductivity meter are available by writing to the authors at
Department of Chemistry, California State University, San Bernardino, CA
92407, or see the online supplement to this article (refer to Supplement link on this page).
Literature Cited
1. Russo, T. J. Chem. Educ.
1986, 63, 981.
2. Mercer, G. D. J. Chem. Educ.
1991, 68, 619.
3. Ghatee, M. H. J. Chem. Educ.
1993, 70, 944.
4. Havrilla, J. W. J. Chem.
Educ. 1991, 68, 619.
5. Solomon, S.; Fulep-Poszmik, A. J. Chem.
Educ. 1991, 68, 160.
6. For another audio conductivity apparatus, see: Katz, D. A.;
Willis, C. J. Chem. Educ. 1994, 71, 330.
7. Williams, H. P.; Cuccaro, L. J. Chem.
Educ. 1990, 67, 788.
|
Citation
