Physics Photo of the Week

February 18, 2011

Meissner Effect - Photo and video by Kyle Brown
Discussion by Owen Kroening and Kyle Brown
Superconductivity, a property shared by most metals, occurs when the temperature of a metal is depressed to a point at which the metal generates no electrical resistance.

While this phenomenon was first observed at liquid helium temperatures (boiling point: 4 K) using mercury metal; it has since been observed at liquid hydrogen temperatures (boiling point: 20 K) using other metals and some alloys.  Much more recently a new class of superconducting materials was discovered consisting of a crystalline-structured, ceramic composite (YBa2Cu3O7).  This material will become superconducting at the temperature of much cheaper liquid nitrogen (boiling point: 77 K).

In these photos, a magnet can be seen floating above a disk of
YBa2Cu3O7 material, which has been cooled using liquid nitrogen. This phenomenon, known as the Meissner effect, discovered in 1933 by Walther Meissner and Robert Ochsenfeld, displays a unique property inherent in superconductors where external magnetic fields are expelled in the superconductive state. The reason for this expulsion is due to the superconductor’s tendency to create electric currents at its surface, which effectively cancels any applied field within the superconductor. Because this expulsion does not change over time, the observed effect remains constant, as the produced currents do not decay, so long as the superconductor remains below critical temperature. The name ‘superconductor’ appropriately refers to this ever-sustaining conductive ability.

In a normal state (above critical temperature), a superconductor will not exhibit the effective expulsion of weak magnetic fields, but allow the fields to penetrate as is normal with other conductors.  As seen in the video here, the magnet rests on the black superconducting disk.  The temperature is above the transition temperature and the superconductor is in its normal state.   Once the temperature of the superconductor cools below the transition temperature, the repulsion of the external field created by the small magnet becomes observable and the magnet levitates.



Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu. 

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