Astronomical observations are suggesting that the fine structure constant
varies cosmologically. We present an analysis on the consequences that these
variations might induce on the electromagnetic field as a whole. We show that
under these circumstances the electrodynamics in vacuum could be described by
two fields, the ``standard'' Maxwell's field and a new scalar field. We provide
a generalised Lorentz force which can be used to test our results
experimentally.Comment: 7 pages, no figures. Accepted for publication in Rev. Mex. Fis. (Some
extra information included, references added and small corrections made to
the original version