Eötvös' Balance

Plate 32 speaks for itself: “Snakewood and platinum fall alike.”

The Physics

Eötvös used a torsion balance to compare how gravity and Earth's rotation act on different materials. Any difference between inertial and gravitational mass would twist the beam; none was seen.

η < 10⁻⁸

• η: fractional difference in acceleration of two materials, dimensionless

1889: equal to 1 part in 20 000 000. Full paper 1922; work shown in 1909.

Honest caveat: The η < 10⁻⁸ bound is conservative: the 1889 work reached 1 part in 2×10⁷ (5×10⁻⁸), and the 1922 Eötvös–Pekár–Fekete paper tightened it to a few parts in 10⁹.

Sources

• R. v. Eötvös, D. Pekár, E. Fekete, Ann. Phys. 373, 11 (1922).
• C. M. Will, Living Rev. Relativ. 17, 4 (2014).

On the poster

Plate 032, “Eötvös' Balance”: the pixel-art plate with its slogan “Snakewood and platinum fall alike.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.