The Little–Parks effect was discovered in 1962 by William A. Little and Roland D. Parks in experiments with empty and thin-walled superconducting cylinders subjected to a parallel magnetic field. It was one of the first experiments to indicate the importance of Cooper-pairing principle in BCS theory. The essence of Little–Parks (LP) effect is slight suppression of the superconductivity by persistent current. Explanation The electrical resistance of such cylinders shows a periodic oscillation with the magnetic flux piercing the cylinder, the period being h/2e ≈ where h is the Planck constant and e is the electron charge. The explanation provided by Little and Parks is that the resistance oscillation reflects a more fundamental phenomenon, i.e. periodic oscillation of the superconducting Tc. The Little–Parks effect consists in a periodic variation of the Tc with the magnetic flux, which is the product of the magnetic field (coaxial) and the cross sectional area of the cylinder. Tc depends on the kinetic energy of the superconducting electrons. More precisely, the Tc is such temperature at which the free energies of normal and superconducting electrons are equal, for a given magnetic field. To understand the periodic oscillation of the Tc, which constitutes the Little–Parks effect, one needs to understand the periodic variation of the kinetic energy. The kinetic energy oscillates because the applied magnetic flux increases the kinetic energy while superconducting vortices, periodically entering the cylinder, compensate for the flux effect and reduce the kinetic energy. Thus, the periodic oscillation of the kinetic energy and the related periodic oscillation of the critical temperature occur together. The Little–Parks effect is a result of collective quantum behavior of superconducting electrons. It reflects the general fact that it is the fluxoid rather than the flux which is quantized in superconductors. The Little–Parks effect can be seen as a result of the requirement that quantum physics be invariant with respect to the gauge choice for the electromagnetic potential, of which the magnetic vector potential A forms part. Electromagnetic theory implies that a particle with electric charge q travelling along some path P in a region with zero magnetic field B, but non-zero A (by ), acquires a phase shift , given in SI units by In a superconductor, the electrons form a quantum superconducting condensate, called a Bardeen–Cooper–Schrieffer (BCS) condensate. In the BCS condensate all electrons behave coherently, i.e. as one particle. Thus the phase of the collective BCS wavefunction