

How Henry Cavendish Used a Wire to Measure a Tiny Force of Gravity - adenner
http://www.scientificamerican.com/article/how-henry-cavendish-used-a-wire-to-measure-a-tiny-force-of-gravity/

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ISL
Our physics research group performs modern precision experimental tests of the
theories of gravity and particle physics. Our workhorse technology remains the
torsion balance.

[http://www.npl.washington.edu/eotwash/experiments](http://www.npl.washington.edu/eotwash/experiments)

There's no other method that can suspend so much material on such a weak
spring with such tiny susceptibility to outside disturbances. Our experiments
benefit from other technical developments from the last 300 years of
experimental science, but it's like comparing modern aircraft to the Wright
Brothers' first flight. The core ideas remain the same.

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PeterWhittaker
I did a similar experiment in second year: We had an enclosed case with two
small balls on either end of a rod which was suspended by a thin wire; outside
the case were two large balls on a rod that pivoted about a bolt in the bottom
centre of the case.

On the wire in the case was a mirror on which we shined a laser, which
reflected to the far wall where we had placed metresticks from one wall to the
other.

Place the big balls against glass, leave, come back after a week when things
had settled down.

Observe where the laser was pointed.

Very quickly pivot the big-ball-rod so that the balls went from front-left-
and-back-right to front-right-and-back-left.

This causes the small balls, which were also front-left-and-back-right, to
swing to front-right-and-back-left, moving the mirror.

Observe where the laser ends up (maximum deflection).

From this, determine G, the gravitational constant. (We knew the masses of the
balls, etc.).

Hardest part of the experiment? Eliminating electrical effects: Grounding the
balls, the glass, etc.

I took weeks to get reliable measurements....

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batbomb
Yeah I did that experiment too, on a Saturday. it blows super hard. you can
see the effects of People running up and down the stairs. if you stand within
a meter too long at the right position, you can distort the gravitational
force. If you bump the case too hard readjusting the balls, you blow it. It
took something like 1 hour for a full cycle? You don't realize all of your
mistakes until 4 hours later.

