Experiment #26 · Scientific experiment

The Cavendish Experiment

Weighing the Earth — and confirming Newton

Henry Cavendish · 1798 · Classical mechanics, foundations of gravity

First published: H. Cavendish, "Experiments to determine the Density of the Earth", *Phil. Trans. Roy. Soc.* 88 (1798): 469–526.

A torsion balance detects the gravitational pull between lead spheres in a basement laboratory — confirming Newton at tabletop scale and yielding the first measurement of *G*.

Cavendish suspended a horizontal rod with two small lead spheres at its ends from a fine torsion wire; two large lead spheres placed near them produced a measurable gravitational attraction, deflecting the rod through a tiny angle. From the deflection and the period of the torsion oscillator, Cavendish extracted the gravitational constant *G* — and thus, knowing Earth's radius, its density. The experiment is the first laboratory demonstration of gravitational attraction between ordinary bodies (as opposed to celestial or planetary motion), and it remains the basis for all modern *G* measurements. Philosophically: it confirms that gravity is universal, not merely a feature of large bodies, and it raises the still-open question of what gravity *is* — a force, a field, or a curvature.

Formulation

Torsion balance: rod (length 1.8 m) with two 0.73-kg lead balls suspended from a silvered copper wire. Two 158-kg lead spheres positioned near the small balls produce a torque. Measured deflection ≈ angular twist proportional to GM₁M₂/r². Cavendish's result: ρ_Earth = 5.448 g/cm³ (modern value: 5.514). G ≈ 6.74 × 10⁻¹¹ N·m²/kg² (modern: 6.674).

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Dimensions Engaged

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Matter

Confirms Matter · Locality (in the qualified Newtonian sense): all matter exerts gravitational force; the attraction is not confined to celestial bodies but extends to laboratory objects.

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Space

Indirectly tests Space · Ontological Status by demonstrating action-at-a-distance through empty laboratory space, raising the question Newton himself left open: what mediates gravity?

Responses — How Schools Engage

Affirms / takes the bait 5

Realism

Scientific realism: gravity is a real force between masses, measurable directly in the laboratory. *G* is a genuine constant of nature, not a fitting parameter.

Naturalism

A canonical example of experimental ingenuity extending the reach of theory: Newton's law, derived from astronomical data, is shown to hold at the scale of metres and kilograms.

Relationalism

A direct measurement of a relation between masses; no further metaphysical anchor in absolute space is required. The result is structural — about how masses are related, not where they "are."

Structuralism

*G* is the canonical example of a structural constant — a numerical value that enters relations without bearing any further metaphysical content.

Logical Positivism

Pure operational physics: a fine torsion balance, a deflection, a number. The experiment exemplifies how physics should establish its constants.

Reframes the question 1

Occasionalism

For a Malebranchean occasionalist, the measured attraction is not a power inhering in matter but a lawlike regularity by which God acts. The experiment measures the regularity, not its cause.