A comparison between He II when the normal fluid is clamped and the superconducting state of metals leads to an extension of the two-fluid equations of London. The following consequences are discussed: (a) A nonstationary thermal emf (thermal electric effect) can exist in a superconductor, (b) a stationary potential difference (not an emf) can exist.

• Received 8 December 1969

DOI:https://doi.org/10.1103/PhysRevLett.24.50