Editing Gyroscope (section)

=== London moment ===
A [[London moment]] gyroscope relies on the quantum-mechanical phenomenon, whereby a spinning [[superconductor]] generates a [[magnetic field]] whose axis lines up exactly with the spin axis of the gyroscopic rotor. A magnetometer determines the orientation of the generated field, which is [[Interpolation|interpolated]] to determine the axis of rotation. Gyroscopes of this type can be extremely accurate and stable. For example, those used in the [[Gravity Probe B]] experiment measured changes in gyroscope spin axis orientation to better than 0.5 [[Minute of arc|milliarcseconds]] (1.4{{e|-7}} degrees, or about {{val|2.4|e=-9|u=radians}}) over a one-year period.<ref>[http://einstein.stanford.edu/content/fact_sheet/GPB_FactSheet-0405.pdf Einstein.stanford.edu] {{webarchive|url=https://web.archive.org/web/20110514044333/http://einstein.stanford.edu/content/fact_sheet/GPB_FactSheet-0405.pdf |date=14 May 2011 }}. "The GP-B instrument is designed 
to measure changes in gyroscope spin axis orientation to better than 0.5 milliarcseconds (1.4x10-7 degrees) over a one-year period"</ref> This is equivalent to an [[angular separation]] the width of a human hair viewed from {{convert|32|km|mi|sp=us}} away.<ref>{{cite web|url=http://history.msfc.nasa.gov/gravity_probe_b/GravityProbeB_20050400.pdf|title=Gravity Probe B – Extraordinary Technologies|url-status=dead|archive-url=https://web.archive.org/web/20100527111732/http://history.msfc.nasa.gov/gravity_probe_b/GravityProbeB_20050400.pdf|archive-date=27 May 2010|access-date=18 January 2011}}</ref>

The GP-B gyro consists of a nearly-perfect spherical [[Moment of inertia#Rotational symmetry|rotating mass]] made of [[fused quartz]], which provides a [[dielectric]] support for a thin layer of [[niobium]] superconducting 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 [[Revolutions per minute|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.<ref>{{cite web|url=http://einstein.stanford.edu/TECH/technology1.html#gyros|title=Gravity Probe B – Extraordinary Technologies|website=Einstein.stanford.edu|access-date=5 November 2017|url-status=dead|archive-url=https://web.archive.org/web/20110514043657/http://einstein.stanford.edu/TECH/technology1.html#gyros|archive-date=14 May 2011}}</ref>

A sensitive [[SQUID#DC SQUID|DC SQUID]] that can discriminate changes as small as one quantum, or about 2 {{e|-15}} [[Weber (unit)|Wb]], 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 pickup 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 torque on the rotor.<ref>{{cite book|pages=44–45|title=Vortex Electronics and SQUIDs|first1=Takeshi|last1=Kobayashi|first2=Hisao|last2=Hayakawa|first3=Masayoshi|last3=Tonouchi|date=8 December 2003|publisher=Springer |isbn=9783540402312|url=https://books.google.com/books?id=5mPeUu1i5R8C&q=dc+squid+reduce+lorentz+force&pg=PA44|url-status=live|archive-url=https://web.archive.org/web/20150904042759/https://books.google.com/books?id=5mPeUu1i5R8C&pg=PA44&lpg=PA44&dq=dc+squid+reduce+lorentz+force&source=bl&ots=Vgz9jQ-IyI&sig=KN71efttIEUKdd63LWfmhO33p90&hl=en&sa=X&ei=wiuYVfHbK4vSoATv55OgDQ&ved=0CC8Q6AEwBQ#v=onepage&q=dc%20squid%20reduce%20lorentz%20force&f=false|archive-date=4 September 2015}}</ref><ref>{{cite web|url=https://www.researchgate.net/publication/234292394|title=DC electrostatic gyro suspension system for the Gravity Probe B experiment|website=ResearchGate|url-status=live|archive-url=https://web.archive.org/web/20150705144927/http://www.researchgate.net/publication/234292394_DC_electrostatic_gyro_suspension_system_for_the_Gravity_Probe_B_experiment|archive-date=5 July 2015}}</ref>