Making a liquid conductor

You will need the following materials to do this investigation:

1. Powdered graphite. This can be found at a hardware store and is used as a lubricant for locks.  It is carbon that has been finely ground from a commonly occurring rock called graphite.  This is a non toxic substance like charcoal.

2. Isopropyl alcohol. This is found at a drug store and is called "rubbing alcohol." This kind of alcohol is unlike that found in beer, wine, or hard liquors. It is poisonous and should never be injested, although it is harmless on the skin. It has an alcohol smell and does evaporate, so make sure to use this in an area where there is plenty of fresh air circulation. We won't be using very much, but it is still good to use caution.

3. White school glue.  Any general purpose, fast drying school glue will work.

4. You will also need:

    • stirring sticks

    • small mixing containers (25 ml plastic hospital portion cups are ideal)

    • cardboard or tag board

    • masking tape

    • pen/pencil

    • small paintbrush

                                            STEP 1

Pour out a 5 ml dry measure of graphite into the mixing cup. Add a small amount of isopropyl alcohol and stir.  Add enough alcohol to create a liquid that is about the consistency of ketchup.

STEP 2

Add about 3 ml of white glue to the mixture and stir.  Watch the mixture as it starts to form a creamy consistency.  This mixture contains carbon atoms that will be held together by the polymer in the glue. The alcohol keeps the mixture liquid until it evaporates leaving a dried conductive residue.

STEP 3

Set the conductive liquid aside for a moment and prepare a index card, piece of tag board, or cardboard as shown. Use masking tape to create three or more tracks on the surface of the cardboard.  In the example shown, we selected 2, 5 and 9 mm widths for these tracks. The track is the space between the masking tape.

                                               STEP 4

Paint on the liquid conductor so that the tracks between the tape lines are covered. Try to maintain an even depth of the fluid between the tape lines.

STEP 5

After the chemical mixture dries, carefully remove the tape. What remains are strips of graphite and dry polymer glue on the card.  You have made chemical resistor strips.

To test the chemical resistance you made, there are two possible routes, one uses a dynamic test that will give qualitative results, and the other gives specific values as tested by an ohmmeter. Either or both methods will suffice:

DYNAMIC TESTING METHOD

You can test your painted-on resistances by using a nine volt battery and an LED.  Set up the circuit as shown below and test each width and length of resistance. Record the results on a data chart such as the one below.

    Brightness of LED with Painted Resistances

For the resistances we made, the data looked like this:  Brightness of LED with Painted ResistancesCHALLENGE:  Using your own generated data from your liquid resistor formula, what width and length would be needed to paint on a satisfactory resistance in the circuit to light the LED at a normal to bright level?  Too much electron flow and the device could be damaged. Too little flow and the LED will not be bright enough. Test the dimension you have predicted based on your data by making a new card with the liquid resistor and testing it.  Report your results and accuracy in design and manufacturing.

THE OHMMETER METHOD

We can measure the actual value of resistance using an ohmmeter. There are several very reasonable electronic ohmmeters available.  The one shown below comes from Harbor Freight Tools, a wholesale tool company. This model can be found on sale for $3-4.  http://harborfreight.com

Every meter like this has several ranges of resistance marked with an omega (Ω) which is the measure of ohms. The larger the value of ohms, the more resistive the material is to electron flow.  Low values of resistors are in the range of hundreds to thousands of ohms whereas large values are in hundreds of thousands to millions of ohms. Remember that the Greatbatch circuit was suppose to have a 10,000 ohm resistor and he had replaced it with a 1,000,000 ohm value. 

The meter in the video below has ranges of 200, 2000, 20k (20,000), 200k (200,000) and 2000k (2,000,000) ohms.  Experiment with the meter you have and measure resistances for each of your painted resistors.  Use a data table to record your information such as:

            Values of Painted Resistances