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Testing the conductivity of solutions can give important insights into the nature of the solutions and the particles in them. Traditionally, conductivity devices used to test the conductivity of a solution or solid usually consist of expensive devices such as volt-ohm meters or specific conductivity meters. Less expensive devices can be constructed that use the brightness of one or several 120-V light bulbs to indicate conductivity. These, devices can be dangerous because they usually have exposed electrodes, presenting the possibility of electric shock or electrocution at 110 V with current up to 15 A. Although commercial versions use shielded electrodes, the high voltage and current remain.
EVOLUTION OF IMPROVED APPARATUS
Russo (1) showed how to construct a simple and safe conductivity apparatus using a blinking light emitting diode (LED) and a 9-V battery. This apparatus is safe due to the low voltage with no possibility of electric shock. The apparatus is portable, so it can be used in any classroom or on a field trip. Further, the apparatus is inexpensive enough to provide several units for a class. There are only two drawbacks: It provides a crude quantitative measurement of conductivity, and the blinking-type LED's can malfunction easily.
Willis (2) made the device a bit mare quantitative by using two LED's, one red and one green. Willis noted that the green LED requires a slightly higher voltage than the red LED. He increased this further using a 100-Omega resistor in series with the green LED.
Katz attempted to duplicate Willis' apparatus but found that the red LED was easily shorted out, Katz modified the circuit by adding a 1000-Omega resistor with a switch to prevent shorting the electrodes during storage. Katz also provided for replaceable or interchangeable electrodes by using screw terminals for attaching the electrodes, which are fitted with spade lugs on one end. (Heavy pencil leads, 1-mm diameter or larger, can be substituted for the copper electrodes.)
A Safe and Accurate Conductivity Tester
A schematic diagram of the conductivity tester is shown in Figure 1, and a parts list is given in Table 1. (Figure 1 omitted) The prototype was constructed on a predrilled circuit board, but any construction technique can be used. For student use, the switch can be omitted and the...