GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

3.2.3.9 Gyro Readout: Marking the UnmarkableFigure 3-18. Schematic diagram and photo of a SQUID gyro readoutChallenge: How does one measure the direction of spin of a perfect unmarked sphere to 0.1 milliarcseconds(3x10 -8 degrees) without disturbing it?Solution: The answer comes through the phenomena of superconductivity. Superconductivity provides a spinpointer which is neither optical nor mechanical as in conventional gyroscopes but magnetic. It also provides asensitive non-interfering instrument to read the pointer, the Superconducting Quantum Interference Device(SQUID).Superconductors have the property that when cooled below a specific temperature they completely lose theirelectrical resistance, hence their name. They become not just great conductors, but for steady currents, perfectones. Additionally, when a superconductor—niobium, for example—spins, it generates a magnetic field (knownas the London moment after physicist Fritz London) which is exactly aligned with its spin axis. Each circlingpositive charge bound in the metal lattice generates a magnetic field parallel to the rotation axis. The total fieldproduced would be very large if not for the superconducting electrons themselves responding to it. Nearly allare forced to keep up with the lattice, generating a cancelling field of their own except in a shallow surface layerabout 100 atoms thick. In this layer they lag slightly behind the charged lattice leaving a small difference field—the London moment—proportional to spin rate and exactly aligned with the spin axis. Here is our pointer.To measure the spin direction, the rotor is encircled with a thin superconducting loop connected to a SQUID(Superconducting QUantum Interference Device) magnetometer. As the orientation of the spin axis of the gyrochanges, the London moment follows, changing minutely the contribution of the London moment to themagnetic field through the loop. So sensitive is the SQUID that a field change of 5 x 10 -14 gauss(1/10,000,000,000,000 of the Earth's magnetic field), corresponding to a gyro tilt of 1 milliarcsecond, should bedetectable within 10 hours, and a 0.1 milliarcsecond deflection should be detectable in 42 days. Feedbackelectronics in the SQUID readout system allow for cancellation of reaction currents in the pick-up loop, thusreducing reaction torques on the gyro to an absolutely negligible level.The London moment readout scheme is well suited to Gravity Probe B. It is sensitive, stable, applicable to aperfect unmarked sphere and does not exert any significant reaction force on the gyroscope.Gravity Probe B — Post Flight Analysis • Final Report March 2007 87