Engineering Physics

9.4 Penetration depth

In 1935, F. London and H. London obtained an expression for penetration of applied magnetic field into superconducting material from the surface by adding: (i) Meissner effect i.e., the magnetic induction (B) inside a superconducting material is equal to zero (B = 0) and (ii) zero resistivity i.e., the intensity of electric field (E) in a superconductor in superconducting state is equal to zero (E = 0) to Maxwell’s electromagnetic equations. According to them, the applied magnetic field does not drop to zero at the surface of the superconductor [in superconducting state] but decreases exponentially as given by the equation:

where H is the intensity of magnetic field at a depth x from the surface, H₀ is the intensity of ...