if we vary trajectories of electrical charges, trajectories of magnetic charges, vector-potential A$_{e}$ and pseudovector-potential B$_{µ}$ (compare, for example, with [7], § 23 and § 30). Integration in the first and fourth terms in the right-hand part of equation (23) is performed with respect to the world lines between two given events, in the rest terms integration is performed with respect to four-dimensional volume; variations of functions are assumed to be equal zero on the integration domain boundary. To vary trajectories of electrical charge e$_{a}$ , we use the expression, describing the current density of this charge via the delta-function of three- dimensional argument: is used to vary the magnetic charge trajectories; a similar expression for the current density of a magnetic charge Because of (15b) density of current j$_{µ}$ is a pseudo-vector and, in consequence of (25), the magnetic charge should be a pseudo-scalar. According to (9), the sign of pseudo-scalar depends on the chosen coordinate system (this can be immediate seen from the expression for the force, acting on magnetic charge in magnetic field) and, hence, the pseudo-scalar 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 using pseudo-tensor F$_{B}$$_{µ}$ to describe an electromagnetic field generated by magnetic charges, i.e. everything, in the issue, depends on the transformation properties of electrical and magnetic fields under extended Lorentz group, consisting of Lorentz transformations, spatial rotations and inversions (changes of sign of basic vectors). To take into account the effect on electrical charges of the electromagnetic field generated by magnetic charges, some effective vector-potential A$_{µ}$ (corresponding to the electromag- netic field, generated by magnetic charges) is added to the four-dimensional vector potential A$_{e}$ in A and a similar term, B$_{e}$ is added to the pseudovector-potential, B$_{µ}$ . Effective potentials A$_{µ}$ and B$_{e}$ are described by equations ∂ $^{i}$Ak µ − ∂ $^{k}$Ai µ ≡ F ik A$_{µ}$ = F ⋆ ik B$_{µ}$ = 1 2 e $^{iklm}$F$_{B}$$_{µ}$$_{lm}$ , (27) j i e = ∑ a j i $_{e$_{a}$}$, j i µ = ∑ b j i $_{µ$_{b}$}$. (26) j i µ$_{b}$ = e$_{b}$δ ( r − r$_{b}$ ) dx i dt (25) j i e$_{a}$ = e$_{a}$δ ( r − r$_{a}$ ) dx i dt , (24) 7