ABSTRACT
The currents in the superconductor were due to electrons described by a wave function which was essentially the same as in the absence of the current. While the simple pairing approximation gives an accurate account of the response of the system to transverse electromagnetic fields, it does not in general give the correct response to longitudinal fields. Among the many surprising properties of superconductors, their response to externally applied electric and magnetic fields is the most striking. In the presence of the Coulomb potential, which of course always exists in real metals, the Bogoliubov-Anderson mode is pushed up to high energy and becomes the plasma oscillation of the electron system. If only the electronic spin polarization is important in the shift, the Knight shift gives a measure of the electronic spin susceptibility. In the original BCS paper it was suggested that there might be low-lying collective spin wave states in a superconductor which give rise to the observed Knight shift.
