Conductivity tester

To construct the conductivity tester you will need:
● a 12-volt AC adapter
This converts the 110-volt electricity from a wall socket to safer 12-volts. It must be 12 volts AC, not DC, because DC will not work for this. You may have a suitable adapter around the house from an old device you're no longer using, or you may get one from an electronics store (e.g. Radio Shack, catalog number273-1631).
● an audio cable with a 1/4-inch or 1/8-inch monaural plug on one end
The plug will become the probe for testing conductivity. You may have an unused cable around the house. What is on the other end does not matter because it will be removed. You may also get a suitable plug-and-cable assembly from an electronics supply store (e.g., Radio Shack, catalog number 42-2381).
● a 12-voltflashlight bulb and socket
The bulb will provide a visible indication of how well a material conducts electricity. You can get these from an electronics store (e.g., Radio Shack, catalog numbers 272-1143 for the bulb and 272-357 for the socket).
● a block of wood about 4 by 4 by 1 inch
The electrical connections will be made on this block, and the lamp will be mounted on it, too.
● two 1-inchwood screws
These hold the lamp socket to the block of wood.
● one 3/4-inch round-headed screw and washer
These will be used to make an electrical connection.
● wire cutter and wire stripper
These are used to prepare the electrical connections.
● a screw driver
Cut the plug from the end of the cord of the AC adapter. Separate about four inches of the cord into its two conductors.Remove about 1 inch of insulation from each of the conductors.
Cut the cord of the audio cable about 2 feet from the plug. Remove about four inches of insulation from the cut end of the cable. This will expose bare stranded wire wrapped around insulation that covers a center wire. Unwrap the stranded wires from the insulation and twist the strands together to make a single bundle. Strip about 1 inchof the inner insulation from the center wire. Use wood screws to attach the lamp base (socket) to the block of wood. Put the washer on the round-head screw and screw it into the block next to the lamp base, but do not tighten the screw yet.
Wrap one wire from the AC adapter (it doesn't matter which) around the screw above the washer. Wrap the end of the bundled wire from the audio plug aroundthe same screw. Tighten the screw to fasten the two wires together.
Attach the remaining wire from the AC adapter to one of the terminals of the lamp base. Attach the remaining wire from the audio plug to the other terminal of the lamp base.
Screw the 12-volt flashlight lamp into the lamp base.
To make the connections more secure, you can use a heavy staple to hold each of the two wires to thewooden block.
The conductivity tester is now complete and ready to use. To test that it works properly, plug the AC adapter into an AC outlet. The lamp will not light. Touch the audio plug sideways to a piece of metal, such as a coin. When the two metal conductors of the plug are shorted by the coin, the lamp will glow brightly. The bright glow indicates that current is easily flowing through thepiece of metal.
Put some water into a cup. Insert the end of the audio plug into the water. If you use distilled water, the lamp will not glow. If you use tap water, the lamp may glow dimly, if at all. If it glows, it shows that the tap water conducts electricity only poorly. Add some table salt to the water and stir the mixture. The lamp will glow brightly when the plug is put into the solution,because salt solution conducts electricity very well, almost as well as metal.
You can investigate different materials from around your house to see how well they conduct electricity when mixed with water. Some things to try, in addition to salt, are sugar, baking soda, shampoo, laundry detergent, rubbing alcohol, and antacid tablets. Anything that dissolves in water can be tested. In order to...