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The London moment (after Fritz London) is a quantum-mechanical phenomenon whereby a rotation superconductor generates a magnetic field whose axis lines up exactly with the spin axis."Towards a new test of general relativity." Physorg Retrieved 10 March 2011 The term may also refer to the magnetic moment of any rotation of any superconductor, caused by the lagging behind the rotation of the object, although the field strength is independent of the charge carrier density in the superconductor.
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 superconductor material. To eliminate friction found in conventional bearings, the rotor assembly is centered by the electric field from six electrodes. After the initial spin-up by a jet of helium which brings the rotor to 4,000 RPM, the polished gyroscope housing is evacuated to an ultra-high vacuum to further reduce drag on the rotor. 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. Einstein.stanford.edu
A sensitive DC SQUID magnetometer able to discriminate changes as small as one quantum, or about , is used to monitor the gyroscope. 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 Induction loop fixed to the housing, inducing a small electric current. The current produces a voltage across a shunt resistance, which is resolved to spherical coordinates by a microprocessor. The system is designed to minimize Lorentz force torque on the rotor. Einstein.stanford.edu
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