Newton's Universal Law of Gravitation is touted as one of the great achievements in our understanding of Nature because it correctly describes the astrophysical behaviour of objects like stars and planets. But the great triumph on Newton's part was realizing that all objects in the Universe, no matter their size, are gravitationally attracted to one another. Gravity is not just of concern for stars and planets!

The mutual gravitational attraction of all objects is usually an overlooked fact, especially in our everyday lives, where the gravitational pull of the Earth on you is far greater in size than the gravitational pull of common every day objects, such as the computer monitor you are currently reading this on.

This presents us with a beautiful conundrum -- as scientifically minded folks, we like to demonstrate our understanding of the principles of Nature by conducting experiments to test our theoretical ideas. So in the Universal Law of Gravitation, how can I possibly design an experiment to test the mutual attraction between two small objects, like apples (~0.5 kg each), when they are in the presence of an object which is a trillion-trillion times more massive (the Earth)?

The solution was happened upon by Henry Cavendish about 100 years after Newton published the Universal Law of Gravitation. Cavendish proposed an experiment (which now bears his name), where two masses were suspended on a balance bar hanging from a fine thread. The thread counters the strong gravitational pull of the Earth, and small forces in the horizontal direction become obvious because they make the apparatus rotate. The basic idea is shown in the figure below.


The Demo

The torsion balance was built from a styrofoam beam suspended using some nylon line to form a cradle, with a length of 6 lb test fishing line as the primary suspension. On the bottom of the beam, I've included a protruding aluminum blade hacked from a can, that rests in a shallow pan of water. This serves as a "water brake" to damp out oscillations around the thread, and bring the system into equilibrium more quickly.

The masses used in this experiments are small bottles of steel shot BBs. I was initially worried about using steel shot since magnetic forces can easily overwhelm the small gravitational effect we are seeing, but I couldn't find enough lead shot to work with. I've convinced myself that the number of BBs is such that the overal magnetic coupling should be neglible owing to randomized orientations. I should do this again with non-magnetic masses.

To minimize the influence of other forces, I did this in an enclosed storage room of my basement, where I sealed as many of the cracks around the doors as I could. I placed a high cardboard barricade around the experiment in an attempt to mitigate air currents. I could watch the experiment remotely (as Cavendish originally watched his experiment remotely) using a Logitech Webcam Pro 9000 attached to my MacBook, with the movie sequences being captured using iStopMotion 2 software from Boinx.

When I set the experiment up initially, I let the thread hang out for a couple of days, and let the balance find its equilibrium direction. I then placed two boards nearby to put the attractive masses on, and waited for the system to stabilize again. Once it was stable, I put the attractive masses in place, and let physics have its way!

Here are a few images of the setup. Click on pictures for a larger view!

materials thumb calibrate thumb beam thumb
balance thumb cut Can thumb brake thumb
ladder thumb setup thumb boxed thumb
laptop thumb

The inspiration and basic design of the Cavendish Experiment presented here has its heritage in an implementation designed by John Walker at Fourmilab, and his Bending Spacetime in the Basement experiment.


Video of the entire explanation and experiment. Double click to start videos.

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