View source for Physics:London moment

{{Short description|Effect of a rotating superconductor}}
The '''London moment''' (after [[Biography:Fritz London|Fritz London]]) is a quantum-mechanical [[Philosophy:Phenomenon|phenomenon]] whereby a [[Physics:Rotation|spinning]] [[Physics:Superconductor|superconductor]] generates a [[Physics:Magnetic field|magnetic field]] whose 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>
The term may also refer to the [[Physics:Magnetic moment|magnetic moment]] of any [[Physics:Rotation|rotation]] of any [[Physics:Superconductor|superconductor]], caused by the [[Physics:Electron|electron]]s lagging behind the rotation of the object, although the field strength is independent of the [[Physics:Charge carrier density|charge carrier density]] in the superconductor.

==Gravity Probe B==
A [[Physics:Magnetometer|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 [[Physics:Gravity Probe B|Gravity Probe B]] experiment measured changes in gyroscope spin axis orientation to better than 0.5 milliarcseconds (1.4{{e|&minus;7}} degrees) over a one-year period.<ref>[http://einstein.stanford.edu/content/fact_sheet/GPB_FactSheet-0405.pdf Einstein.stanford.edu]</ref> This is equivalent to an angular separation the width of a human hair viewed from 32 kilometers (20 miles) away.<ref>{{Cite web |url=http://history.msfc.nasa.gov/gravity_probe_b/GravityProbeB_20050400.pdf |title=History.msfc.nasa.gov |access-date=2011-10-03 |archive-url=https://web.archive.org/web/20100527111732/http://history.msfc.nasa.gov/gravity_probe_b/GravityProbeB_20050400.pdf |archive-date=2010-05-27 |url-status=dead }}</ref>

The [[Physics:Gravity Probe B|GP-B]] gyro consists of a near-perfect spherical [[Physics:Moment of inertia#Rotational symmetry|rotating mass]] made of [[Chemistry:Fused quartz|fused quartz]], which provides a [[Physics:Dielectric|dielectric]] support for a thin layer of [[Chemistry:Niobium|niobium]] [[Physics: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 [[Physics: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|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 Einstein.stanford.edu]</ref>

A sensitive DC [[Physics:SQUID|SQUID]] magnetometer able to discriminate changes as small as one [[Physics:Magnetic flux quantum|quantum]], or about {{val|2|e=-15|ul=Wb}}, is used to monitor the gyroscope. A precession, or tilt, in the orientation of the rotor causes the London moment [[Physics:Magnetic field|magnetic field]] to shift relative to the housing. The moving field passes through a superconducting [[Engineering:Induction loop|pickup loop]] fixed to the housing, inducing a small electric current. The current produces a voltage across a [[Engineering:Shunt (electrical)|shunt resistance]], which is resolved to [[Spherical coordinate system|spherical coordinates]] by a microprocessor. The system is designed to minimize [[Physics:Lorentz force|Lorentz]] [[Physics:Torque|torque]] on the rotor.<ref>[http://einstein.stanford.edu/TECH/technology1.html#readout Einstein.stanford.edu]</ref>

==Magnetic field strength==
The magnetic field strength associated with a rotating superconductor is given by:
:<math>B=-\frac{2M }{Q}\ \omega ,</math>
where ''M'' and ''Q'' are the mass and the charge of the superconducting charge carriers respectively.<ref>{{cite journal
  | last = Brady | first = R. M.
  | url=https://www.cl.cam.ac.uk/~rmb4/PhD-2London.pdf
  | title = Correction to the Formula for the London Moment of a Rotating Superconductor
  | archive-url=https://web.archive.org/web/20160429032603/https://www.cl.cam.ac.uk/~rmb4/PhD-2London.pdf
  | archive-date=2016-04-29
  | year = 1982
  | journal = [[Physics:Journal of Low Temperature Physics|Journal of Low Temperature Physics]]
  | volume = 49  | issue = 1  | pages = 1–17
  | doi =  10.1007/bf00681758
  | bibcode = 1982JLTP...49....1B | s2cid = 123534732
 }}</ref> For the case of [[Physics:Cooper pairs|Cooper pairs]] of electrons, {{nowrap|1=''M'' = 2''m''<sub>e</sub>}} and {{nowrap|1=''Q'' = 2''e''}}. Despite the  electrons existing in a strongly interacting environment, ''m''<sub>e</sub> denotes here the mass of the bare electrons<ref>{{cite journal
  | last = Tate| first = J.
  | title = Determination of the Cooper-pair mass in niobium
  | year = 1990
  | journal = [[Physics:Physical Review B|Physical Review B]]
  | volume = 42  | issue = 13  | pages = 7885–7893
  | doi =  10.1103/PhysRevB.42.7885
  | pmid = 9994948
  |bibcode =   1990PhRvB..42.7885T|display-authors=0
 }}</ref> (as in vacuum), and not e.g. the [[Physics:Effective mass (solid-state physics)|effective mass]] of conducting electrons of the normal phase.

==Etymology==
Named for the physical scientist [[Biography:Fritz London|Fritz London]], and [[Physics:Moment|moment]] as in [[Physics:Magnetic moment|magnetic moment]].

==See also==
* [[Physics:Barnett effect|Barnett effect]]

==References==
{{Reflist}}


{{DEFAULTSORT:London Moment}}
[[Category:Quantum mechanics]]

{{Sourceattribution|London moment}}