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| The '''London moment''' is a [[quantum-mechanical]] [[phenomenon]] whereby a [[rotation|spinning]] [[superconductor]] generates a [[magnetic field]] whose [[Axis of symmetry|axis]] lines up exactly with the spin axis.<ref>"Towards a new test of general relativity." http://www.physorg.com/news12054.html. Physorg. Retrieved 10 March 2011</ref>
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| The term may also refer to the [[magnetic moment]] of any [[rotation]] of any [[superconductor]], caused by the [[electron]]s lagging behind the rotation of the object, although the field strength is independent of the [[charge carrier density]] in the superconductor.
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| ==Gravity Probe B==
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| 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) over a one-year period.<ref>http://einstein.stanford.edu/content/fact_sheet/GPB_FactSheet-0405.pdf</ref> This is equivalent to an [[angular separation]] the width of a human hair viewed from 32 kilometers (20 miles) away.<ref>http://history.msfc.nasa.gov/gravity_probe_b/GravityProbeB_20050400.pdf</ref>
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| The [[Gravity Probe B|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]] [[superconductor|superconducting]] material. To eliminate friction found in conventional bearings, the rotor assembly is centered by the electric field from six electrodes. After the initial [[Angular acceleration|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>http://einstein.stanford.edu/TECH/technology1.html#gyros</ref>
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| A sensitive DC [[SQUID]] magnetometer able to discriminate changes as small as one [[Magnetic flux quantum|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 [[Induction loop|pickup loop]] fixed to the housing, inducing a small electric current. The current produces a voltage across a [[Shunt (electrical)|shunt resistance]], which is resolved to [[spherical coordinate system|spherical coordinates]] by a microprocessor. The system is designed to minimize [[Lorentz force|Lorentz]] [[torque]] on the rotor.<ref>http://einstein.stanford.edu/TECH/technology1.html#readout</ref>
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| ==Magnetic field strength==
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| The [[magnetic field strength]] associated with a rotating superconductor is given by:
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| :<math>B=-\frac{2M }{Q}\ \omega </math>
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| where ''M'' and ''Q'' are the mass and the charge of the superconducting charge carriers respectively.<ref>{{cite journal
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| | last = Brady | first = R. M.
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| | title = Correction to the Formula for the London Moment of a Rotating Superconductor
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| | year = 1982
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| | journal = [[Journal of Low Temperature Physics]]
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| | volume = 49 | issue = 1 | pages = 1–17
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| | doi = 10.1007/bf00681758
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| |bibcode = 1982JLTP...49....1B }}</ref> For the case of [[Cooper pairs]] of electrons, ''M=2m<sub>e</sub>'' and ''Q=2e''. Despite the electrons existing in a strongly-interacting environment, ''m<sub>e</sub>'' denotes here the mass of the bare electrons <ref>{{cite journal
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| | last = Tate et al. | first = J.
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| | title = Determination of the Cooper-pair mass in niobium
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| | year = 1990
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| | journal = [[Physical Review B]]
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| | volume = 42 | issue = 13 | page = 7885
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| | doi = 10.1103/PhysRevB.42.7885
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| |bibcode = 1990PhRvB..42.7885T}}</ref> (as in vacuum), and not e.g. the [[Effective mass (solid-state physics)|effective mass]] of conducting electrons of the normal phase.
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| ==Etymology==
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| Named for the physical scientist [[Fritz London]], and [[moment (physics)|moment]] as in [[magnetic moment]].
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| ==See also==
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| * [[Barnett effect]]
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| ==References==
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| {{Reflist}}
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| {{Wiktionary}}
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| {{DEFAULTSORT:London Moment}}
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| [[Category:Quantum mechanics]]
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