Building a Circuit to Test Electrical Conductivity

Electricity flows through our homes and schools in metal wires (which are conductors), but it won’t flow through certain materials (insulators), such as plastic or wood. Here we’re going to find out what other materials may be conductors or insulators. Also make sure to mention that this is a circuit, and electricity needs a complete circuit through which to flow. If the circuit is broken (the leads aren’t connected properly, etc), then no electricity will flow and the light bulb won’t light.

No extra safety precautions are required.

1 battery
1 battery holder
1 flashlight bulb
1 bulb holder
3 alligator clips
Cardboard
Metal thumbtacks or brads
Coins
Keys
Cork
Buttons
Fabric
Screws
Marbles
Plastic
String
Water/salt (see below in notes)
Any other materials whose conductivity you want to test

Place the battery in the holder and attach an alligator clip to each of the leads on the battery holder.
Connect the light bulb, in its holder, to one of the wires that is attached to the battery. Connect another alligator clip to the other lead on the light bulb holder.
Cut a piece of cardboard roughly 2 inches by 2 inches. In the center of the cardboard insert two metal thumbtacks or paper fasteners about ½ inch away from each other. Connect one of the remaining alligator clips, one leading from the battery and one leading from the light bulb, to each of the thumbtacks. An open circuit has been created, with the gap between the thumbtacks serving as the break in the circuit.
Test various materials for their conducting ability by using them to bridge the gap between the thumbtacks. Conductors, such as coins, lying across the gap in contact with the thumbtacks should make the light bulb light. Insulators will not because they do not complete the circuit.

It may be helpful to describe an open vs. closed circuit. Also, if possible, acquire/bring some DI water and show the students that water by itself does not conduct electricity, and then add some salt to it and show that it’s the salt/ions in the water that actually conduct.

Q: Why does the bulb light when some things are used to connect the tacks, but not others?
A: The materials which allow the light bulb to light up are conductors and allow the electrons to flow through them. The other materials are insulators and don’t let the electrons flow.
Q: What materials made the bulb light? Could you guess which other materials would be conductors or insulators?
A: Glass, plastic, metal filament, metal wiring.
If you have time, you can give a brief explanation of the battery making the electrons flow in terms of its positive and negative ends.

(none at this time)

Put the materials away, making sure to remove the batteries from the holder. Storing batteries in the holder will drain them more quickly, and the batteries also might leak acid (yuck).