This is my first time posting here, I apologize if this is the wrong place to ask such a question. In my book I have the following London equation written (1st) for a superconductor: E=μ0λ2L∂J/∂t where: λ2L is the london penetration depth. My understanding is that it can be derived from newtons 2nd law, by simply assuming the electron is accelerated indefinitely, and writing in terms of current densities. My issue with this is that there is an ernous factor of 1/4 that turns up in the final answer which is sometimes included in λ2L that I cannot resolve. The wikipedia article here: (http://en.wikipedia.org/wiki/London_penetration_depth) does not include the factor of 1/4 in λ2L. I can't find anywhere to help with this inconsistency, I can only think that the mass is half of that in the drude model (after the scattering term is removed) or the charge is a factor of √2 greater.