Pure water does not conduct electricity very well. However, when some materials are dissolved in water, the solution is a very good conductor of electricity. When other things are dissolved, the solution may conduct poorly, or not at all. You can build a conductivity tester that uses a light bulb to indicate how well a solution conducts electricity. Here's how.

You will need the following materials:

a 12-volt A.C. transformer

This transformer plugs into a household electrical outlet and reduces the voltage from 120 volts to 12 volts A.C. Make sure the transformer produces A.C., not D.C. A D.C. power supply won't work. A suitable transformer can be obtained at a Radio Shack store, the catalog number is #######.

a 12-volt light bulb and socket

You can get this from Radio Shack, too. The catalog numbers are ##### for the bulb and ##### for the socket.

a 12-inch piece of 12 gauge, single strand, insulated copper wire

This you can get from a hardware store.

two round-head 3/4 inch wood screws

You can get these at the hardware store, or a lumber yard.

a block of wood

A six-inch piece of 1 by 2 is ideal; you can get this from the lumber yard.

adhesive tape

Electrical tape or duct tape will do nicely.

In addition to these materials, you will need some tools: a wire stripper, a flat-blade screw driver, and pliers.

Here's how to assemble the tester.

1. Cut the 12-inch piece of insulated, single-strand, 12-gauge copper wire into two 6-inch pieces. Strip 1 inch of insulation from one end and 1/4 inch of insulation from the other end of each wire.
   
2. Screw the two screws into the block of wood, about 1/2 inch from the end of the block and about 1 inch apart. Before tightening the screw, wrap the 1-inch section of bare copper wire around the screw. A pair of pliers can help with this.
   
3. Attach the second wire to the other screw in the block of wood. Mount the light-bulb socket onto the block of wood near the screws.
   
4. Remove the connector from the end of the cable of the transformer. Separate about six inches of the two wires at the cut end of the cable. Cut a 4-inche piece off the end of one of the wires, and save this piece. Strip about 1 inch of insulation from the end of the two wires of the transformer. Attach the shorter wire to one terminal of the light-bulb socket. Attach the longer wire to one of the screws holding a 12-gauge wire.