The Meissner Effect
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.
What is Superconductivity?
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.
