Abstract
We propose a dynamical explanation of the Meissner effect in superconductors and predict the existence of a spin Meissner effect: that a macroscopic spin current flows within a London penetration depth λL of the surface of superconductors in the absence of applied external fields, with carrier density = the superfluid density and carrier speed 
(me= bare electron mass). The two members of a Cooper pair circulate in orbits of radius 2λL in opposite direction and the spin current in a Cooper pair has orbital angular momentum ℏ. Our description also provides a "geometric" interpretation of the difference between type-I and type-II superconductors.