Precision spheres for the Gravity Probe B experiment

Spherical gyroscope rotors for a fundamental experiment to test two predictions of General Relativity Theory, geodetic precession and frame dragging, were manufactured out of fused quartz and single-crystal silicon. These rotors required a mass unbalance and asphericity of less than 25 nm (1 μin), a fractional difference in the moments of inertia of less than 10−5, and a diameter within 0.5 μm (20 μin) of a standard sphere with a diameter of 37.996 mm (1.4959 in). We describe the manufacturing process and the associated metrology.

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