Conductivity Meter

1. From the string of holiday lights, cut off an individual light. Two wires should extend from the base of the small bulb—trim each wire to a length of about 2 inches (5 centimeters) from the base of the light. Then, using a wire stripper or scissors, expose 1–2 cm of the interior metal on each wire.
   
2. Wrap some tape around the top edge of the battery (the end with the terminals). This will reduce the chance of short-circuiting the battery by insulating the wires from the battery’s metal casing.
   
3. Wrap a small piece of aluminum foil around one of the exposed wires running from the light bulb, then gently push the foil-wrapped wire end into the positive terminal of the battery. Use the tape to secure and completely cover over the inserted foil-wrapped wire.
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4. Roll small pieces of aluminum foil into two tubes. These will be your “leads” to test the conductivity of a solution. (Stainless steel wire can also be used instead of aluminum foil to reduce corrosion and reactions with the solution being tested.) 
   
5. Gently push one end of an aluminum lead directly into the negative battery terminal and secure it with tape.
    
6. Wrap the other aluminum lead around the bulb's remaining exposed wire.
   
7. Attach your assembly to the craft stick (or other nonconductive flat object) by laying the battery on the craft stick and securing it with tape. Use extra tape if necessary to ensure that the aluminum leads won't detach from the battery or the end of the light bulb. Allow the aluminum leads to dangle from the craft stick, and leave some of the stick exposed at both ends of the device. This will enable the device to rest on top of the solution containers. Test your device to ensure that it works properly by briefly touching the aluminum leads together. The bulb should light up.
   Note: Don’t let the bulb burn too long with direct current from the 9-volt battery. It will burn out the bulb.
    
8. Option: Because of chemical activity, your aluminum foil leads will decompose over time in the solutions you test. To extend the life of your device, you can attach the aluminum foil leads to two stainless steel screws. This can be done by twisting the foil leads around the screws. Only the screws, not the foil portion, should come in contact with the solutions you're testing. 
9. Separate the leads: your device is ready.

Test if a solution conducts electrical currents:

Fill a cup with salty water or other ionic solution, and then rest the device on the rim of the cup (make sure the aluminum leads are immersed in the solution). If the light bulb glows brightly, then the solution is very conductive. If it barely glows—or not at all—then your solution is not very conductive.

Test a series of solutions (try solutions with varying concentrations of electrolytes):

Fill each cup to the same height with solutions containing varying amounts of salt. Rest the device on the rim of each cup, one after the other (make sure the leads are immersed in the solutions at the same depth—this will keep the contact area the same for each solution tested). Compare how the light bulb glows in each of the solutions.

Test to see if a chemical reaction is occurring:

Fill a cup with a salt solution and then place your device on the rim of the cup. Watch the aluminum leads and see what develops. Do you notice a color change or gas at one of the aluminum leads? Do you see your solution getting cloudy or detect an odor emanating from the cup? These and other subtler features may indicate that a chemical change or reaction is happening.

If your solution lights the bulb, it contains electrolytes. Electrolytes are charged particles, usually ions in solution. These ions can have positive or negative charges. Ions are attracted to their corresponding oppositely charged lead: Positively charged particles move and are attracted to the negative aluminum lead and negatively charged particles move and are attracted to the positive aluminum lead. This flow of charged particles completes the circuit, causing the light bulb to glow.

Often, these moving ions can cause interesting chemical phenomena to result. Ions may come out of solution, interact with the aluminum metal, cause gas bubbles to form, decompose the foil, and change the pH of the solution. You may observe several chemical changes and reactions in your cup of solution.