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My son Max is investigating which materials are the best conductors. He built a circuit with a 6V battery and a light bulb, hoping to distinguish which materials are the best conductors by the brightness of the light bulb. The experiment worked, but the results were not satisfactory to Max. The ligh bulb was either on or off, and it was difficult to tell if one material was better than the other. He asked if there was a way to measure the conductivity of materials with another instrument. I purchased a multimeter from Radio Shack hoping to measure the conductivity/resistance of the materials. However, I am not sure I am properly operating the instrument. We removed the test material from the circuit and set the multimeter to the OHMS setting. The results for the metals (copper, aluminum foil, a nickel, and a penny) bounce all over the place and often end at zero, and the non-conductors do not show any change on the screen so the end result is also zero.I would really like to help Max find more specific results, but I am not sure what else to do. Is there a particular bulb that would show more variety in the intensity? He has shown such a curiosity about this. I sincerely hope there is a way to help him! Thank you for your time. Max's mom.
Answer 1:

Standard multimeters (like the one you bought from Radio Shack) are usually used to test the resistance of resistors in electric circuits. Metals are too good of a conductor for them to register a resistance on a multimeter like that, which is why you're getting erratic readings (and insulators, which don't conduct at all, will just give you zero).

There are a few options I can suggest. If Max wants to measure the conductivity of metals, he'll need to use very, very thin, long wires to be able to get a result and notice a difference between different types of metal (the longer and thinner a wire is, the greater its resistance). You might be able to find wires like this at Radio Shack, or probably at a crafts store like Michaels in the jewelry section. Ask for high gauge wires (the thickness of a wire is its gauge, and very thin wires have a higher gauge. If you can find some 40 gauge wire, and use a piece a few feet long, the multimeter should be able to give you a good reading).

But it might be easiest to just use some different substances. For example, here's a fun experiment Max could try: have Max use a #2 (or softer) pencil to draw a long, thin solid rectangle on a piece of paper (maybe one by ten inches in size), and color it in very heavily with the pencil. Pencil lead is made of graphite, which is a good but not excellent insulator, so the rectangle he's drawn will be a resister. Then have Max tape one wire from his light bulb device to one end of the rectangle, and have him touch the other wire to the other side of the rectangle. The light bulb should grow brighter and dimmer as he moves the wire along the rectangle (here's a quick description of a similar experiment, with pictures):

pictures

He could also try things like testing the conductivity of lemon juice, water with different amounts of salt in it (this might be easiest to do if you soak a piece of paper towel with them), the conductivity of fruits and veggies, etc. Basically, anything that isn't obviously a very good conductor or a very good insulator should give decent results.

I hope this helps! And feel free to write back if you need more suggestions or ideas.



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