The Meissner Effect

The Meissner effect is one of the most interesting occurrences resulting from superconductivity. Discovered by Walter Meissner and Robert Ochsenfled, they noticed that a superconducting magnet will repeal the magnetic flux lines of an applied magnetic field. (Fig. 2). Thus, as the above video shows, repealing a magnet and causing it to ‘levitate’ above the superconductor. This is the principle by which the Japanese ‘Maglev’ trains work.

The magnetic field is zero in the core of a superconductor, although the magnetic field does penetrate a small distance (known as the London Penetration Depth). The Meissner effect is present in all known superconductors and is considered a defining characteristic.

Fig. 2 Magnetic Flux Lines In a non and Superconducting Material

                                     What is Superconductivity?

Superconductivity is a phenomenon exhibited by some elements, alloys and compounds

whereby, below certain temperatures, they show no electrical resistance. As well as this
it was discovered that super conductors expel magnetic flux lines in a phenomenon called the ‘Meissner Effect’

In a conventional electrical conductor resistance drops as temperature decreases but never reaches zero. In a superconducting material resistance also drops with temperature until a point known as the ‘Critical Temperature’ (T c ). At this point the electrical resistance suddenly drops to zero. (Fig 1) . As a general trend the Tc's of metallic superconductors such as Lead or Mercury are under 10 K, while those of intermetallic compounds and alloys are in the region of 10-25K. However a new second type of superconductor exists, so called Type II or High Temperature superconductors which have achieved Tc's of 150K.

The Importance of this zero electrical resistance is that one does not waste energy when carrying a current, none of the electrical flow is converted into heat due to the resistance. Also it means in a round superconductor a current can be induced which will carry on going round, in theory, infinitely. The closest thing to perpetual motion, yet discovered, in nature.

Meissner Effect Video