This paper is addressed to those who apply Hertz’s Electrodynamics to moving media, this being the only Galilean relativistic electrodynamics, similarly to the classical Mechanics. The purpose of this paper is to prove how the electric flux law must be applied in the study of the devices with moving media. In order to exemplify this, we have chosen the unipolar induction devices. If dS is an element on the surface Σ within a medium moving with respect to the laboratory system of reference at one point on dS, the electric flux density is D or D0, depending on how is defined or measured: by the observer ℜ within the laboratory system of reference, or by the local observer ℜ0, on dS within a system moving toghether with the medium. There are differences between the electric fluxes S D d ⋅ ∫ Σ and S D d 0 ⋅ ∫ Σ . The Maxwell’s Electrodynamics, extended for moving media, using only quantities defined by ℜ, is the classical macroscopic Electrodynamics, which explains away all the experiments with moving media, including those of unipolar induction. In Maxwell-Hertz Electrodynamics, still used in engineering, the electric flux is S D d 0 ⋅ ∫ Σ . This Electrodynamics cannot explain all the results obtained by unipolar induction experiments.

Keywords: Electromagnetic quantities in moving media, Electric flux, Unipolar induction, Distribution of electric charge.