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

Our physics research group performs modern precision experimental tests of the theories of gravity and particle physics. Our workhorse technology remains the torsion balance.<p><a href="http://www.npl.washington.edu/eotwash/experiments" rel="nofollow">http:&#x2F;&#x2F;www.npl.washington.edu&#x2F;eotwash&#x2F;experiments</a><p>There&#x27;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&#x27;s like comparing modern aircraft to the Wright Brothers&#x27; first flight. The core ideas remain the same.

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.<p>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.<p>Place the big balls against glass, leave, come back after a week when things had settled down.<p>Observe where the laser was pointed.<p>Very quickly pivot the big-ball-rod so that the balls went from front-left-and-back-right to front-right-and-back-left.<p>This causes the small balls, which were also front-left-and-back-right, to swing to front-right-and-back-left, moving the mirror.<p>Observe where the laser ends up (maximum deflection).<p>From this, determine G, the gravitational constant. (We knew the masses of the balls, etc.).<p>Hardest part of the experiment? Eliminating electrical effects: Grounding the balls, the glass, etc.<p>I took weeks to get reliable measurements....