| 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.
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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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