Faraday's law of induction is emf = - (d/dt) ∫S B∙da . When the closed loop (serving as the boundary of the surface S) is independent of time, the above relation is equivalent to the Maxwell equation curl E = - ∂B/∂t . However, when the closed loop C (i.e. the boundary of S) is itself a function of time, the following two questions seem relevant to ask: (i) How is the first equation above to be applied? (ii) Is this method consistent with Galilean (or Lorentz) invariance?