Physics Photo of the Week
Meissner Effect - Photo and video by Kyle Brown
Discussion by Owen Kroening and Kyle Brown
property shared by most metals, occurs when the temperature of a
metal is depressed to a point at which the metal generates no
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).
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
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.
Photo of the
published weekly during the academic year on Fridays by the Warren
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