In the conventional stacking method of the laminated core of a transformer, grain-oriented silicon steel sheet pieces of an identical grade or the identical magnetic properties are used as legs and yokes of the core. In a highly oriented silicon steel sheet having a high B8 value due to excellent magnetic properties in the rolling direction of the sheet, the excellent magnetic properties cannot be fully utilized for the watt loss reduction in the conventional stacking method. In a laminated core of the present invention, the transformer core comprises a grain-oriented silicon steel sheet having a higher orientation used for a leg(s) and a grain-oriented silicon steel sheet having a lower orientation used for a yoke(s). The former steel has the B8 value of generally 1.88 Tesla or higher and preferably 1.89 Tesla or higher. The latter steel has the B8 value of generally 1.86 Tesla or lower. A low watt loss, particularly at a low or medium magnetic flux density, can be achieved by the present invention, while decreasing the cost of the transformer.
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