nature of magnetic charges can be considered as a proof of their existence impossibility (this issue will be, possibly, discussed later together with quantization of charges). The pseudo-scalar nature of magnetic charges, in a given case, is a direct result of transformation properties of electrical and magnetic fields under extended Lorentz group, consisting of Lorentz transformations, spatial rotations and inversions (changes of sign of basic vectors). To take into account the effect on electrical charges of the electromagnetic field, generated by magnetic charges, some effective vector-potential A$_{g}$ (corresponding to the electromag- netic field, generated by magnetic charges) is added to the four-dimensional vector potential A$_{e}$ in A and a similar term B$_{e}$ is added to the pseudovector-potential B$_{g}$ . Effective poten- tials A$_{g}$ and B$_{e}$ are described by equations Using the effective potential, we replace in equations the external electromagnetic field, generated by magnetic (electrical) charges, which affects an electrical (magnetic) charge, by the same electromagnetic field, generated by fictitious electrical (magnetic) charges; the requirement for such replacement is that magnetic (electrical) charges must be absent in the space-time region of interest [electromagnetic field must not have singularities due to magnetic (electrical) charges]: respectively. Because we do not consider particles bearing both electrical and magnetic charges, this requirement is always met locally, in the close vicinity of electrical (magnetic) charge. As we use the variational principle to derive Lagrange-Euler differential equations, i.e. locally, we may assume, that these requirements are met. If requirement (30) is met, external derivation of antisymmetric four-dimensional tensor of the second rank F ik A$_{g}$ ∂ $^{i}$Ak g − ∂ $^{k}$Ai g def = F ik A$_{g}$ = F ⋆ ik B$_{g}$ = 1 2 e $^{iklm}$F$_{B}$$_{g}$$_{lm}$ , (28) ∂ $^{i}$Bk e − ∂ $^{k}$Bi e def = F ik B$_{e}$ = F ⋆ ik A$_{e}$ = 1 2 e $^{iklm}$F$_{A}$$_{e}$$_{lm}$ . (29) j i g ( x ) ≡ 0 (30) j i $_{e}$( x ) ≡ 0 , (31) ∂F ik A$_{g}$ ∂x$_{l}$ + ∂F kl A$_{g}$ ∂x$_{i}$ + ∂F li A$_{g}$ ∂x$_{k}$ , (32) or 8