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by Procheta Mallik


Now that you've played around with some conductors and insulators, this guide explore the idea of electrical resistance and changing the electrical resistance of a material. (Water in this case.)

    • Scissors have functional sharp edges. Contact may result in injury.

    • Always keep blades away from fingers and body. Handle with care.

  1. To make the battery holder please visit this link DIY Battery holder - Foam and Spring.
    • The longer leg is the positive terminal.

    • Connect the wire from the negative terminal of the battery to negative terminal of the LED.

    • Attach another wire to the positive end of the LED. Keep the other end free.

    • Remove insulation from both the ends of the wires before you connect to LED or battery.

    • Keep the wire from the positive end of the battery free.

    • We are going to use the free ends to check the conductivity of different materials.

    • You can also keep a longer wire ready to attach it on the positive end. This is only useful for checking bigger objects.

    • The wires are now your test leads.

    • Use these test leads to test various materials for their conductivity. Never bring the test leads in contact with each other; keep them at least 2-3cm apart on every object that you test.

    • If the LED does not glow, it is an example of a negative result, in this case the material is an insulator.

    • If the LED glows, it is an example of a positive result, in this case the material is a conductor.

    • The connections between the battery and safety pin might be loose, in that case hold the two terminals with your thumb and index finger for a better connection.

    • Make sure all the connections are clean and stable, otherwise the current will not flow.

    • To ensure proper connection, wind the wire several times over the material.

    • Does the brightness of the LED vary among different conductors?

    • Conductivity is directly proportional to brightness of the LED. (True/ False)

    • Are all metals conductors, and all non-metallic substances insulators? (Yes/No)

    • Can an LED be connected in any direction and still work? Can you think of any other use for this property of the LED?

    • Why is it important to connect the batteries as we did here? Are there other ways of connecting multiple batteries?

    • Use the observation table shown alongside to classify the materials into good and bad conductors and insulators.

    • Good conductor - LED glows brightly. Bad conductor - LED glows dimly. Insulators - LED does not glow.

    • Try to check some bigger materials like iron rod, bicycle wheel rim, cricket bat etc.

    • Try to check the conductivity of liquids like milk, curd, lemon juice or any other fruit juice.

    • If you want to check the conductivity of even more materials, please click on this link Electrical Conductivity (Variations).

    • Setup up the conductivity tester as shown in the picture, with about 4cm of of space between the leads.

    • Now fill the bottle cap half way with tap water.

      • You can try distiled water as well, if available.

    • Now carefully place the cap in-between the test leads and bend them till both are touching the water's surface but not touching each other.

    • The led will either not light at all or be very dimly inuminated depending on various factors such as water purity, battery voltage and led efficiency.

      • In the case of distilled water the led will not light at all because of the high resistance of pure water.

    • Now add about a teaspoon of salt (5grams) and observe the led.

    • The led will slowly rise in brightness as the salt dissolves in the water and decrease its resistance. The led will not get as bright as with a good conductor like a metal because the water still maintains a resistance to current flow.

    • You can make the salt dissovle faster by stirring the water with a match stick or straw for about 30 seconds.


Resistance to the flow of electricity can be a very useful property. The method of dimming a load (a light or fan) by placing a resistance in series with it, is how all early dimmers for electrical appliances worked. Some of the excess energy is lost as heat across the resistor. Even in our case, a tiny amount of heat is being lost through the water, changing its temperature. This change is so small in the case of an LED that it's quite tricky to measure, but old-style dimmers for mains use used to get quite hot indeed. It would be interesting to find out why this happens.

Kailash NR

Member since: 05/02/2017

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