Eureka : Discovering Your Inner Scientist by Orzel Chad

Author:Orzel, Chad [Orzel, Chad]
Language: eng
Format: mobi
ISBN: 9780465044917
Publisher: Perseus Book Group
Published: 0101-01-01T00:00:00+00:00

THE CAVENDISH EXPERIMENT

The experiment that made Cavendish’s name was based on another of Newton’s laws, in this case, his universal law of gravitation. According to this law, every object with mass in the universe attracts every other object with mass in the universe. This force has a simple and elegant mathematical form, which depends on the two masses and the square of the distance between them (so, doubling the distance between two masses cuts the force to a quarter of its original value). One of Newton’s greatest accomplishments was showing that this mathematical form completely explains the orbits of planets in the solar system.

While Newton’s laws were enough to explain the general behavior of the planets, setting the scale of the solar system remained a problem. The orbits of the planets were well known relative to each other, but not the absolute sizes—astronomers had measured the ratio of the radius of Mars’s orbit to the radius of the Earth’s orbit, but not the exact value of either. Numerous methods were employed to try to get the scale, most successfully observations of the transit of Venus, when the planet passes in front of the Sun as seen from Earth. The precision of these measurements was limited, in part, by the gravitational attraction of the mountains that were, then as now, often the site for astronomical observations. This force could slightly deflect the plumb lines that astronomers used to establish the vertical, introducing some error.

The scientific societies of the day, particularly Britain’s Royal Society, mounted expeditions to “weigh” mountains and thus quantify their gravitational effect, but the necessary measurements were difficult to make and interpret. In an effort to simplify the process, a new experiment to “weigh” the Earth was launched by John Michell and Henry Cavendish. The idea of the experiment was simple: to measure the gravitational force between two objects of known mass and compare that force to the weight of those same objects due to the gravitational attraction of the Earth. Combined with Newton’s law of gravitation, this would give a measurement of the mass of the Earth, which could be converted to a density to allow more accurate determinations of the force from mountains and other landscape features. Michell started the measurement, but died before it could be completed, so the final experiment was carried out by Cavendish and set the standard for precision measurements for years to come.

Henry Cavendish was descended from one of the wealthiest and most powerful families in England, but following in the footsteps of his father, Lord Charles Cavendish, who resigned from Parliament to devote himself full-time to the Royal Society, Henry pursued a career in science rather than the more traditional dabbling in politics. The younger Cavendish was excruciatingly shy and reclusive, but the Michell-Cavendish experiment to weigh the Earth perfectly suited him, as the entire project could be undertaken without his leaving the grounds of his family home outside London.* The apparatus was moved to Cavendish’s property after Michell’s death in

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