The experiment that tested the equivalence principle by measuring the acceleration of different substances was the Eötvös experiment.
Hungarian physicist Loránd Eötvös developed a highly sensitive torsion balance to compare gravitational and inertial mass. If different materials had different ratios of these masses, Earth’s gravity and the centrifugal effect caused by Earth’s rotation would produce slightly different accelerations. That mismatch would twist the suspended apparatus.
Eötvös began this work around 1885 and used different materials in 1889, reporting no measurable composition-dependent difference at the accuracy then available. Later experiments by Eötvös, Dezső Pekár and Jenő Fekete, including a lengthy series from 1906 to 1909, improved the precision substantially.
The experiment did not directly test whether objects simply fall at visibly different speeds. Instead, it looked for an extraordinarily small difference in acceleration or direction between substances. Its results helped establish the equivalence of inertial and gravitational mass, a central idea later incorporated into Einstein’s general theory of relativity.