The Meissner effect describes the complete expulsion of a magnetic field from the interior of a material as it transitions into its superconducting state. This phenomenon occurs when a material is cooled below its critical temperature, regardless of whether the magnetic field was applied before or after cooling began. Discovered in 1933, this effect demonstrated that superconductors possess a fundamental magnetic property, establishing that they are more than just materials with zero electrical resistance.
Defining the Complete Magnetic Expulsion
The complete expulsion of magnetic flux is a spontaneous event that occurs during the phase transition to superconductivity. German physicists Walther Meissner and Robert Ochsenfeld first observed this phenomenon in 1933 while measuring magnetic fields outside samples of lead and tin. They found that when the temperature dropped below the critical threshold, the internal magnetic field abruptly vanished.
This active exclusion distinguishes a superconductor from a hypothetical perfect conductor. A perfect conductor, defined solely by zero electrical resistance, would simply trap any existing magnetic field lines inside it as it cooled. In contrast, the Meissner effect shows that a superconductor actively pushes out the field lines that had already penetrated the material.
The result is that the magnetic field becomes zero within the bulk of the superconducting material. This behavior effectively makes the superconductor a perfect diamagnet, as it generates an internal magnetization that perfectly opposes the external field.
The Role of Screening Currents in Field Rejection
The mechanism for magnetic field expulsion involves the formation of persistent, dissipationless electrical currents, known as supercurrents. These supercurrents are spontaneously induced on the material’s surface as it cools below its critical temperature. They are called screening currents because they block the external magnetic field from entering the interior.
The flow of these screening currents generates their own internal magnetic field. This induced field is oriented in the exact opposite direction to the external applied magnetic field, causing the two fields to cancel each other out everywhere within the bulk of the superconductor, resulting in a net magnetic field of zero.
The magnetic field strength does not vanish instantly; rather, it decays exponentially as it enters the surface. The characteristic distance over which the external magnetic field significantly decreases is known as the London penetration depth. This depth typically ranges from 50 to 500 nanometers, meaning the screening currents only need to flow within this very thin layer near the surface to achieve expulsion in the interior. The London equations provide the theoretical framework for describing this exponential decay.
Visible Manifestations and Practical Applications
The most recognizable consequence of the Meissner effect is the phenomenon of magnetic levitation. When a magnet is placed near a superconductor, the field expulsion generates a powerful, upward-directed repulsive force. This repulsion is strong enough to allow a magnet to float stably above the cooled superconducting material.
This ability to repel magnetic fields has been translated into several advanced technologies, most notably in high-speed ground transport. Maglev trains utilize powerful superconducting magnets to achieve levitation and propulsion, which eliminates frictional resistance with the track. For example, systems like the Japanese SCMaglev rely on the magnetic forces generated by superconducting coils to lift the train and achieve speeds exceeding 600 kilometers per hour.
The Meissner effect also plays a significant, though less visible, role in medical imaging and scientific instrumentation. Magnetic Resonance Imaging (MRI) machines depend on extremely strong and stable magnetic fields to produce high-resolution images of the body. Superconducting magnets are used in these devices because the zero-resistance state allows for continuous, high-current flow, while the Meissner effect helps maintain a pure and consistent magnetic field environment necessary for clear imaging.
Furthermore, the magnetic shielding capability inherent in the field expulsion is utilized in sensitive measurement tools. Superconducting Quantum Interference Devices, or SQUIDs, are highly sensitive magnetometers that detect incredibly weak magnetic fields. The Meissner effect is leveraged in SQUIDs to create magnetically shielded environments, which is a necessary condition for their extreme sensitivity in detecting subtle magnetic signals, such as those produced by the human brain.