Re: cavendish expt

[Tom Ford <tomford@THESCIENCESOURCE.COM>]

I did this once using broadcast width video tape in a very tall gymnasium.
Air currents were a BIG problem. Just opening and closing one door at
midnight would affect the apparatus. I used 4" diameter solid lead balls
cast in steel shells, two suspended and two on floor stands. A mirror on
the vertical tape reflected a laser back to the wall to amplify the
deflection. Like any sensitive balance, it had to be read on the fly by
noting the stable center of swings for one position of the floor stands and
comparing that with the stable center of swings after the floor stands had
been repositioned. There was about a 15cm difference with a total beam path
of about 30m. The magnetic tape was clamped over a bar at top and bottom.
An attic overhead gave access to the removable light fixture to suspend the
device from the bar resting on the floor above.

My guess is that there is too little mass in the bottles of sand, but there
was a 16mm film from the early PSSC era that showed this being done with
large amounts of sand and electrostatic screening made from window screen
material.

Tom Ford


At 09:00 AM 11/12/02 -0800, you wrote:
> Greetings phys-eders:
>
> I am attempting to set up an experiment similar to
> what Cavendish did when he "weighed the earth".
>
> I'm using two joint compound buckets filled with sand
> as the larger masses. I have a meterstick horizontally
> oriented with bottles of sand on each end. This is
> suspended from the ceiling with video tape.
>             |
>             |
>            /\
>           /  \
>          /    \
>         ________
>         0      0
>
>        OO      OO
>   (In front)  (In back)
>
> I'm finding that it takes an incredibly long time for
> the suspended meterstick to find its equilibrium
> position. Once it does, I'll move the buckets in
> closer to the bottles of sand and see if I can detect
> any torque on the meterstick.
>
> My questions are:
>
> 1)Do you think the incredibly small Fg would produce
> any effect?
>
> 2)Have any of you tried this before? If you did, how
> did it work?
>
> 3) Any suggestions?

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.