How Henry Cavendish Used a Wire to Measure a Tiny Force of Gravity

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.

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....