A torsion-balance comparison of the gravitational acceleration of bodies of different composition, testing equality of inertial and passive gravitational mass up to a universal proportionality factor. A universal acceleration law independent of test-body mass and composition agrees with this weak equivalence principle test. It does not by itself establish the strong equivalence principle for self-gravitating bodies.
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The Eötvös experiment, named after the Hungarian physicist Loránd Eötvös, is a fundamental experiment that tested the equivalence principle, which is a key component of Einstein's theory of general relativity. The equivalence principle states that gravitational mass (the mass that determines the strength of the gravitational force) and inertial mass (the mass that determines how an object accelerates when subjected to a force) are equivalent.