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Re: [Phys-l] FW: universal gravitation lab



Here's another from Sci. Am. (Sept. 1963)


"An inexpensive version of the Cavendish apparatus, modified to include electrostatic shielding, has been constructed and used successfully for evaluating G within 1 per cent of the currently accepted value by Sam Epstein of Los Angeles.


Figure 3: Lower end of suspension
Epstein writes: "Essentially the Cavendish apparatus is a sensitive torsion balance and a balance arm suspended at the middle by a slender steel wire. A small lead weight suspended from each end of the arm is attracted to large weights by mutual gravitation. The rotation of the arm varies with the intensity of attraction and is measured by the deflection of a light beam from a small mirror fixed to a pair of hooks that support the balance arm, as shown in the accompanying illustration below].

"The complete balance assembly including the case is supported from the ceiling by a length of 3/4-inch pipe that terminates in flanges. The structure is braced by a set of three guy wires. The torsion wire attaches at the upper end to a special fitting that rests in the top end of the pipe: a brass flange and cap nut drilled and threaded to receive a small rod with a hook at one end for attaching the wire. The flange is centered in the pipe by four adjusting screws and can be rotated for adjusting the orientation of ..."


what makes these, the above and

http://www.fourmilab.ch/gravitation/foobar/ ,


work is the scale as compared to the Leybold and other miniature apparatuses.
However, here is a smaller apparatus, also rather more high tech:




"The Cavendish experiment is not an ordinary demonstration in physics because it is arduous to do. The arm takes a good deal of time to settle down into its equilibrium position. Any perturbation from the environment (for example from the experimenters' moving around the laboratory) adds further to the settling time. As a result-the demonstration often takes hours. Working from a prototype built by Greg Eibel of Reed College, Crandall has devised a clever electronic version of the Cavendish experiment that takes only minutes and yields a value for the gravitational constant of (7.5 + 1.5) X 10-11N(m2/kg.2).

Whereas the classical Cavendish experiment depends on the rotation of an arm, the experiment devised by Crandall depends on preventing such a rotation. The strength of the gravitational force is measured from the magnetic force that is required to prevent the rotation of a torsion bar when an additional mass is brought close to it. The position of the torsion bar is monitored by an optical sensor."

The above is from an article by Jearl walker (also Sci. Am. Jan 1983). It's essentially a magnetic cybernetic balance similar to the Cahn Electro Balances.


bc

p.s. Crandall's (Reed College) apparatuses include the PE effect and optical Doppler shift in addition to the G measurement.




Brian Whatcott wrote:

At 09:14 AM 2/14/2007, you wrote:

From: WC Maddox

For a cheaper homemade version of Cavendish experiment try this:
http://www.fourmilab.ch/gravitation/foobar/



Now that WAS a physics setup. Torsion to spare in fact.
Which reminds me that I seem to recall that Cavendish discarded
his first copper torsion wire as too compliant, allowing the arm
to hit the draft-box.


Brian Whatcott Altus OK Eureka!

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