- An elementary module of a magnetic core of an electrical transformer of the wound type, made up of a first (1; 2) and second (3; 4) superimposed winding, respectively made from a first and second material, said first material being a crystalline material with a saturation magnetization (Js) greater than or equal to 1.5 T, preferably greater than or equal to 2.0 T, better still greater than or equal to 2.2 T, and magnetic losses of less than 20 W/kg in sinusoidal waves with a frequency of 400 Hz, for a maximum inductance of 1 T, preferably less than 15 W/kg, preferably less than 10 W/kg, said first material being chosen from among Fe-3% Si alloys with oriented grains, Fe-6.5% Si alloys, Fe-15 to 55% total of Co, V, Ta, Cr, Si, Al, Mn, Mo, Ni, W alloys, textured or not, soft iron and ferrous alloys made up of at least 90% Fe and having Hc < 500 A/m, ferritic stainless steels Fe-Cr with 5 to 22% Cr, 0 to 10% total Mo, Mn, Nb, Si, Al, V and with more than 60% Fe, non-oriented electric steels Fe-Si-AI, Fe-Ni alloys with 40 to 60% Ni with no more than 5% total additions of other elements, Fe-based magnetic amorphous materials with 5 to 25% total B, C, Si, P and more than 60% Fe, 0 to 20% Ni + Co and 0 to 10% other elements, all of these content levels being given in percentages by weight. and said second material being a material with an apparent saturation magnetostriction (λ sat ) less than or equal to 5 ppm, preferably less than or equal to 3 ppm, better still less than or equal to 1 ppm, and magnetic losses of less than 20 W/kg in sinusoidal waves with a frequency of 400 Hz, for a maximum induction of 1 T, preferably less than 15 W/kg, preferably less than 10 W/kg, said second material being chosen from among Fe-75 to 82% Ni - 2 to 8% (Mo, Cu, Cr, V) alloys, cobalt-based amorphous alloys, and FeCuNbSiB nanocrystalline alloys, characterized in that the cross-sections (S 1 ; S 2 ) of the first winding (1; 2) and (S 3 ; S 4 ) of the second winding (3; 4) are such that the ratio (S 1 /(S 1 + S 3 ); S 2 /(S 2 + S 4 )) of each cross-section of the first material with a high saturation magnetization (Js) compared to the cross-section of the set of the two materials of the elementary module is comprised between 2 and 40%. and said second material being a material with an apparent saturation magnetostriction (λ sat ) less than or equal to 5 ppm, preferably less than or equal to 3 ppm, better still less than or equal to 1 ppm, and magnetic losses of less than 20 W/kg in sinusoidal waves with a frequency of 400 Hz, for a maximum induction of 1 T, preferably less than 15 W/kg, preferably less than 10 W/kg, said second material being chosen from among Fe-75 to 82% Ni - 2 to 8% (Mo, Cu, Cr, V) alloys, cobalt-based amorphous alloys, and FeCuNbSiB nanocrystalline alloys, characterized in that the cross-sections (S 1 ; S 2 ) of the first winding (1; 2) and (S 3 ; S 4 ) of the second winding (3; 4) are such that the ratio (S 1 /(S 1 + S 3 ); S 2 /(S 2 + S 4 )) of each cross-section of the first material with a high saturation magnetization (Js) compared to the cross-section of the set of the two materials of the elementary module is comprised between 2 and 40%. <EOS>