  Based on an analogy between Fluid Mechanics and Electromagnetism, we claim
that the gauge conditions of Classical Electromagnetism are not equivalent
contrary to the common belief. These "gauges" are usually considered as
mathematical conditions that one must specify in order to solve any
electromagnetic problem. Here, the author shows that these conditions are
physical constraints which can be interpreted as electromagnetic continuity
equations. As a consequence, light cannot be considered as a pure transverse
wave in vacuum from the point of view of the potentials. We discuss the (lack
of) meaning of gauge transformations.
