This instructable includes directions on how to make a simple conductivity meter. I use my meters with my chemistry classes to test the conductivity of different materials.

Materials
plastic film canister
drill
LED and resistor
soldering iron and solder
stiff copper posts
9 V battery cap
9 V battery

Step 1: Preparing the Canister

I used a film canister for the outside housing to make the conductivity meter at least somewhat water resistance, and because I had several of them in the storeroom.

You need to make three small holes in the base of the canister through which you will stick the two probes (lengths of copper) and the glove of the LED. I used a small drill, you could also carefully poke through an ice pick or other small tool. Be careful not to make your holes too large, the fit needs to be snug.

I used a permanent marker to label the canister "conductivity meter" as well.

Step 2: Assembling and Testing the Circuitry

Now assemble your circuit. Any visible light LED will work for this application just make sure you have the appropriate size resistor in series with your LED to protect the LED from bursting when you connect the battery.

Connect these components together in a line (not a closed loop):

one copper probe
resistor
LED
one end of the 9 V battery connector

then to the other end of the 9 V battery connector you solder on the second copper probe.

Before continuing, put a 9 V battery onto the connector and touch the two copper probes together to test the circuit. If the meter is working properly then the LED should remain lite while the copper probes are in contact.

Step 3: Assembling the Meter

Now assemble your meter by inserting the probes through two of the holes in the bottom of the canister, and the LED through the third hole.

Let the wires to the 9 V battery cap come out of the canister at the top and use the cap to hold them in place. Add your battery and you are ready to start testing materials!