History of Physical Science Cavendish and The Weight of The World

Kevin Lakawicz, Jonathan Tatro, Adam Urbanic

The Cavendish Experiment was the first experiment to yield accurate results that laid the path to find the universal gravitational constant. Despite Cavendish often being credited with finding the value of the universal gravitational constant, his experiment actually yielded the Earth’s density. To achieve this, Cavendish used a torsion pendulum, a shockingly sensitive device for the time. Cavendish did not originally design the apparatus, but he modified it to his own exacting specifications. Nor did his experiment yield the value for the universal gravitational constant. This monumental experiment was only achievable by the collaboration of the scientific community of the time, many of who were members of the Royal Society along with Cavendish. Due to the fact that Cavendish had crippling shyness, possibly stemming from mental illnesses, he rarely published his works, even though they later proved to be accurate predictions of future scientific advances and discoveries. Despite this shyness, Cavendish was still involved in the interweaving web of other scholars, building on other’s work and having his work expanded upon and replicated. These same processes continue to this day.

One of the lesser-known facts about Henry Cavendish was that he was incredibly shy and socially awkward. While many famous scientists may have been quirky or odd, Cavendish brought this behavior to a level that makes some historians speculate that he may have had Asperger’s Syndrome. This (weakness, illness, behavior, problem) is sometimes blamed for relatively low number of papers published by Cavendish despite the myriad experiments he preformed. When James Clerk Maxwell later looked through Cavendish’s notes, he discovered that Cavendish had accurately predicted several advances in physics, electricity, and chemistry but was either too shy or too unsure of his work to publish it.

Focus of Study

Cavendish For Undergraduates

Challenges

The Experiment

Faculty Advisors: Dr. McLean, Dr. Cope, Dr. Towsley

Boys, C.V. "On the Newtonian Constant of Gravitation." Philosophical Transactions of the Royal Society of London 186 (1894), http://www.jstor.org/stable/90640 (accessed March 23, 2012).

Cavendish, Henry. "Experiments to Determine the Density of the Earth." Philosophical Transactions of the Royal Society of London 88 (1798), http://www.jstor.org/stable/106988 (accessed February 19, 2012).

"Eulogy on the Late Hon. Henry Cavendish." The Belfast Monthly Magazine 10 (1813), http://www.jstor.org/stable/30074459 (accessed March 8, 2012).

Lidbetter, Hugo. "Henry Cavendish and Asperger’s syndrome: A new understanding of the scientist." Personality, Psychopathology, and Original Minds 46 (2009), http://www.sciencedirect.com/science/article/pii/S0191886909000294 (accessed March 24, 2012).

References

Cavendish set up two large lead balls on alternate sides of the horizontal wooden arm of the balance. Their mutual attraction to two small lead balls caused the arm to rotate, twisting the wire supporting the arm. The arm would stop rotating when it reached an angle where the twisting force of the wire equaled and balanced the gravitational force of attraction between the large and small lead spheres. By measuring the angle of the rod, and knowing the torque of the wire for a given angle, Cavendish was able to determine the force between the masses. Since the gravitational force of the Earth on the small balls could be directly determined by the balls’ mass, the ratio of the two forces allowed the density of the earth to be calculated, using Newton’s laws of gravitation. His experiment found that the Earth's density was 5.448 ± 0.033 times that of water (due to an arithmetic error, found by F. Baily in 1821, Cavendish’s report to the Royal Society contains the value 5.48 ± 0.038).

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