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This Week's Experiment - #315 Franklin's Bells
This week's experiment comes from the bad weather we have had.
Besides the rain, we have also had some very nice lightning. I enjoy
lightning. I watch it every chance I get. Ben Franklin was also very
interested in lightning, and he came up with a very creative, but quite
dangerous method of letting him know that it was time to go watch for
lightning. We will do something similar, but in a very safe way. You will
need:
two aluminum soft drink cans
a wooden or plastic ruler or stick at least 6 inches long
thread
aluminum foil
tape
a television
Ben Franklin wanted a warning that lightning was near, so he put a lightning
rod on his roof and ran a wire from it to a metal bell in his lab. Beside
it was another metal bell and between the bells there was a string with a
metal ball. When the lightning rod developed a static charge, the bell was
charged too. That caused the metal ball to bounce back and forth, ringing
the bells.
We will make a bell that will ring when it is charged with static
electricity. Place the two cans on a flat surface beside the television.
On one can, bend the metal tab which is used to open the can back and forth
several times, until it breaks free. Save this tab. Arrange the cans so
that one is very near the screen of the television and the other is about two
inches away. Tie a piece of thread to the tab you removed. Place the
ruler across the tops of the two cans, as if you were forming a bridge. Tie
the thread around the ruler, so that the metal tab is hanging between the two
cans. The cans will be the bells and the tab on the string will move back
and forth to strike the cans. Now all we need is the static charge. We
will not use anything as dangerous as lightning. We will get our static
charge from the screen of the television.
Turn on the television. If you hold the back of your hand near the screen
of the television, you will probably feel the hair on your hand standing on
end. That is the result of the static charge on the screen. We want to
collect enough of this static charge to ring our bell. To do that, turn off
the TV. Tear a sheet of foil about the size of the screen. Use some tape
to hold the foil in place. Tear a small strip of foil and tape one end to
the foil on the screen. Tape the other end to the nearest can. Make sure
that the tab is hanging between the two cans. It should almost touch both
cans.
Now we are ready to test it. DO NOT TOUCH THE CANS AS YOU TURN THE TV ON
AND OFF. It is not harmful, but you will get a painful static shock that
could scare you. Turn on the television. The tab should clang back and
forth between the two cans a few times. When it stops, turn off the
television and it should clang back and forth a few more times.
Your television produces a picture by shooting a stream of electrons at the
phosphor dots on the screen. Some of these electrons wind up on the screen
as a static charge. The foil collects quite a bit of this charge and
transfers it to the first can. That gives the can a negative charge. The
metal tab starts out neutral, but as soon as the can develops a negative
charge that changes. Since like charges push away from each other, some of
the negatively charged electrons in the tab are pushed away from the
negatively charged can. That leaves a positive charge on the side towards
the can. Unlike charges attract, so the tab is pulled to the can. As soon as
it touches, electrons move from the can to the tab. Now the tab has a
negative charge too. Like charges repel so the tab is pushed away. It hits
the other can, giving up its extra electrons and goes back to being neutral.
Then the whole thing happens again. This keeps happening until the charge
dissipates. Since your television screen has the strongest charge just as
it is turned on and off that is when you get the reaction in your bell.
While this is safe with the tiny amount of electricity you are getting from
your television, it would be incredibly dangerous to try with real lightning.
Any nearby strike could prove fatal. Franklin was tremendously lucky to
live through his experiments. Too bad he did not have a television to make
things safer. Of course, then he probably would have watched movies instead
of cleaning out his truck...I mean instead of performing his experiments.
Have a fantastic week.
These experiments are from Robert Krampf - The Happy Scientist
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