Abstract: A transformer core includes a first core member formed by laminating grain oriented 3% silicon steel sheets and second core members disposed at respective sides of the first core member in the direction of lamination so that the first core member is sandwiched by the second core members. Each second core member is formed by laminating non-oriented 6.5% silicon steel sheets. A commercial frequency component of magnetic fluxes mainly passes through the second core members such that an amount of noise caused by a higher harmonic component is reduced.

Abstract: A method of manufacturing the peripheral wall of a tank for an oil-immersed electric apparatus which comprises manufacturing a panel unit through the steps of:fabricating a panel by pressing a thin steel sheet to form an upward extending flange along both edges of the panel and provide a plurality of reinforcement depressions concaved toward the backside of the flat portion of said panel;folding the panel into two parts with the surface of the panel kept outside along the central line extending between both edges of the panel to fabricate a hollow fin;joining both sides of the folded fin by spot welding the reinforcement depressions; andwelding together the closely facing edges of both sides of the fin,and constructing the peripheral wall of the tank by joining together a plurality of panel units fabricated through the above-mentioned steps by welding the flanges of said panel units.