and a similar term would occur in the right-hand part of equation (4b). This contradicts our initial assumption that the electromagnetic field generated by electrical charges and the electromagnetic field generated by magnetic charges are described independently and  hence  with varying potentials A$_{e}$ and B$_{µ}$ we should not vary the terms of full action  containing the effective potentials. The source-containing terms of equations (4b)  (15b) make no sense to quantization of a free electromagnetic field in relativistic quantum field theory. Since a distinction between action (23) and an ordinary action without magnetic charges is not important  from viewpoint of relativistic quantum field theory  the Feynman rules for the calculation of the interaction cross-sections for elementary particles interacting with electrical or magnetic charges can be formulated by generalizing the known results of rela- tivistic quantum field theory for an electromagnetic field without magnetic charges (see  for example  [8]  § 24). Operator S ( S -matrix)  which relates amplitudes of the initial Φ ( −∞ ) and the final Φ ( ∞ ) states: – can be expressed via chronological exponent where L$_{I}$ ( x ) is Lagrangian of interaction and For the charged particle propagator we have an ordinary expression in terms of function D $^{c}$( x − y ) for scalar particle of mass m$_{n}$ – compare with [8]  (15.17)  (22.9): 9 Φ ( ∞ ) = S Φ ( −∞ )   (30) S = T { exp [ i ℏ ∫ L$_{I}$ ( x ) d $^{4}$x ]}   (31) L$_{I}$ ( x ) = − 1 c 2 ( A k e + A k µ ) j$_{e k}$ − 1 c 2 ( B k µ + B k e ) j$_{µ k}$ (32) j k e = ∑ a ce$_{a}$ ¯ ψ$_{a}$γ $^{k}$ψ$_{a}$   j k µ = ∑ b cµ$_{b}$ ¯ ψ$_{b}$γ $^{k}$ψ$_{b}$ . (33) 〈 T { ψ$_{n}$ ( x ) ¯ ψ$_{n}$ ( y ) } 〉 = − i ( iγ $^{k}$∂$_{k}$ + m$_{n}$c ℏ ) D $^{c}$( x − y ) . (34) III. MAGNETIC MONOPOLE IN RELATIVISTIC QUANTUM FIELD THEORY