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Re: cavendish expt



thomas pfeiffer wrote:

I'm using two joint compound buckets filled with sand
as the larger masses.

Lead would be better.

1) It gets the center of mass closer to where
the action is ... 1/r^2 and all that.

2) All the sand I've ever seen is full of iron
particles. I would be worried about magnetic
systematic errors. It is easy to get Pb free
of impurities.

3) Although lead is more costly than sand, you
don't need huge amounts of it.

I have a meterstick horizontally
oriented with bottles of sand on each end. This is
suspended from the ceiling with video tape.

Videotape seems odd. Monofilament fishing line
is pretty much standard for this application.
Videotape will do bad things to the sensitivity.

I'm finding that it takes an incredibly long time for
the suspended meterstick to find its equilibrium
position.

So add some hydraulic damping.
http://www.fourmilab.ch/gravitation/foobar/

My questions are:

1)Do you think the incredibly small Fg would produce
any effect?

Yes. There shouldn't be a problem due to
lack of signal. The apparatus doesn't have
to be super-tall or super-massive.

> 3) Any suggestions?

It's not hard to find good information on this topic.
http://www.google.com/search?q=cavendish+experiment+air-currents

Note that I didn't simply google "cavendish experiment"
... I added the term air-currents in order to select
sites that discussed the real issues.

The difference between hackers and scholars
is that the scholar checks the literature
before hacking.

=================

It would be amusing to see how small you can
make the apparatus. I'll bet it wouldn't be
too hard to make it smaller than a breadbox.
You can get some very small fibers cheaply
and easily as follows: Go down to the boat
store and get some good Dacron kernmantel
rope. Cut through the outer braid with a razor
knife. Pull. Inside you will find angel
hair, so fine that a single strand can't
be seen unless the light hits it just right.
One strand can only support a few grams. I
haven't measured or calculated the torsional
spring constant, but it must be reeeeally
small.

=========================

Over 350 years ago there appeared a book called
"Discourses on Two New Sciences". One of the new
sciences was the laws of motion. The other one
was scaling laws.

Scaling laws are important. Never pass up
the opportunity to make a scaling-law argument.

It is amusing to see how the sensitivity of the
Cavendish apparatus scales with size. Work it
out. The back of my envelope says if you make
the apparatus 10 times smaller, the raw
sensitivity goes down by only a factor of
sqrt(10). And I don't think raw sensitivity
is the issue -- what you care about is signal
to noise ratio, and your control over noise
sources (air currents etc.) gets better when
you keep it small.

You easily win back that factor of sqrt(10)
by converting from sand to lead, as soon as
the thing is small enough that the mass can
be affordably implemented in lead.