To make a simple device to test the conductivity of objects and find out whether they are good or bad conductors of electricity.
1. A small flashlight bulb.
2. A battery.
3. A shoe box.
4. Three pieces of wire, each around 10 cms long.
5. Duct tape.
6. Different conductors to be tested, such as a metal coin, a plastic comb, a piece of paper, a dish of water, etc.
1. Make a small hole on the top of the shoebox and insert the flashlight bulb into it so that the protruding end goes into the box while the head of the bulb stays outside.
2. Tape the battery onto the bottom of the shoebox on the inside of the box.
3. Tape the end of a wire to the positive end of the battery and another wire to the negative end of the battery.
4. Connect the wire from the positive of the battery to the metal tip of the bottom of the bulb.
5. Tape one end of the third wire to the metal ring on the side of the bulb.
6. Connect the free end of this wire to the free end of the wire connected to the negative terminal of the battery. If the bulb light up, the connections are right and the device is ready.
7. Now connect the free end of the third wire and the wire coming from the negative terminal of the battery to different objects in turn. When connected to the metal coin you will observe that the bulb lights up. When connected to the plastic comb it will not light up. Thus you can distinguish the good conductors from the bad ones. When good conductors are connected to the device the bulb will light up. When the bad ones are connected, the bulb will either not light up or will light up feebly if the object is not a complete insulator.
The flashlight bulb lights up when the circuit is complete with the battery connected. When different objects are connected in the circuit, the circuit is completed only when good conductors are connected. When insulators like the plastic comb are connected, the current does not flow through the circuit and the bulb does not light up.