For example, those used in the Gravity Probe B experiment measured changes in gyroscope spin axis orientation to better than 0.5 milliarcseconds (1.4×10−7 degrees) over a one-year period.
[3] The GP-B gyro consists of a near-perfect spherical rotating mass made of fused quartz, which provides a dielectric support for a thin layer of niobium superconducting material.
Provided the suspension electronics remain powered, the extreme rotational symmetry, lack of friction, and low drag will allow the angular momentum of the rotor to keep it spinning for about 15,000 years.
A precession, or tilt, in the orientation of the rotor causes the London moment magnetic field to shift relative to the housing.
The moving field passes through a superconducting pickup loop fixed to the housing, inducing a small electric current.