An armature for an electric motor or a generator, the armature comprising: - a plurality of split cores mutually connected to form an annular core; and - a winding wire wound on each of the split cores, wherein: - a connection projection and a connection recess are formed on coupling surfaces (21, 31) located on both ends of each of the split cores in a circumferential direction to mutually connect adjacent ones of the split cores, and - in a cross sectional plane perpendicular to a center axis line of the annular core: - the connection projection and the connection recess have complementary configurations in which the connection projection and the connection recess are narrowed in width with distance from the coupling surfaces (21, 31); - each of the connection projection and the connection recess includes a pair of friction surfaces (23, 24; 33, 34) extending in a direction separate from the coupling surfaces (21, 31); characterized in that - one of the pair of friction surfaces (23, 24) of the connection projection is inclined relative to a virtual normal line (PL, PL1, PL2) that is normal to the coupling surfaces (21, 31); - one of the pair of friction surfaces (33, 34) of the connection recess is inclined relative to a virtual normal line (PL, PL1, PL2) that is normal to the coupling surfaces (21, 31); - the other one of the pair of friction surfaces (23, 24) of the connection projection is parallel to the virtual normal line (PL, PL1, PL2), and - the other one of the pair of friction surfaces (33, 34) of the connection recess is parallel to the virtual normal line (PL, PL1, PL2). - a plurality of split cores mutually connected to form an annular core; and - a winding wire wound on each of the split cores, - a connection projection and a connection recess are formed on coupling surfaces (21, 31) located on both ends of each of the split cores in a circumferential direction to mutually connect adjacent ones of the split cores, and - in a cross sectional plane perpendicular to a center axis line of the annular core: - the connection projection and the connection recess have complementary configurations in which the connection projection and the connection recess are narrowed in width with distance from the coupling surfaces (21, 31); - each of the connection projection and the connection recess includes a pair of friction surfaces (23, 24; 33, 34) extending in a direction separate from the coupling surfaces (21, 31); characterized in that - one of the pair of friction surfaces (23, 24) of the connection projection is inclined relative to a virtual normal line (PL, PL1, PL2) that is normal to the coupling surfaces (21, 31); - one of the pair of friction surfaces (33, 34) of the connection recess is inclined relative to a virtual normal line (PL, PL1, PL2) that is normal to the coupling surfaces (21, 31); - the other one of the pair of friction surfaces (23, 24) of the connection projection is parallel to the virtual normal line (PL, PL1, PL2), and - the other one of the pair of friction surfaces (33, 34) of the connection recess is parallel to the virtual normal line (PL, PL1, PL2). - the connection projection and the connection recess have complementary configurations in which the connection projection and the connection recess are narrowed in width with distance from the coupling surfaces (21, 31); - each of the connection projection and the connection recess includes a pair of friction surfaces (23, 24; 33, 34) extending in a direction separate from the coupling surfaces (21, 31); characterized in that - one of the pair of friction surfaces (23, 24) of the connection projection is inclined relative to a virtual normal line (PL, PL1, PL2) that is normal to the coupling surfaces (21, 31); - one of the pair of friction surfaces (33, 34) of the connection recess is inclined relative to a virtual normal line (PL, PL1, PL2) that is normal to the coupling surfaces (21, 31); - the other one of the pair of friction surfaces (23, 24) of the connection projection is parallel to the virtual normal line (PL, PL1, PL2), and - the other one of the pair of friction surfaces (33, 34) of the connection recess is parallel to the virtual normal line (PL, PL1, PL2).