the four poles 13a to 13d have the same structure the following description concerns only the pole 13a as specifically shown in fig4 the pole 13a is divided circumferentially into two sectors 13a 1 and 13a 2 the sector 13a 2 covers a wider angular range than the sector 13a 1 the adjacent ends of the sectors 13a 1 and 13a 2 are connected to the core 12 through links 14a 1 and 14a 2 respectively the links 14a 1 and 14a 2 provide magnetic channels through which almost all the flux flowing from magnetic poles of the magnet 10 through the armature gap to pole 13a pass before reaching the core 12 the density of the magnetic flux is greatest at each of these links a hole 15a is formed at a location generally at the center of the link 14a 2 accordingly the magnetic channel the area having the maximum magnetic flux density for the sector 13a 2 has a smaller cross section than that of the sector 13a 1 this relation is expressed by where t is the core thickness at the links 14a and 14a 2 a 1 is the width of the link 14a 1 a 2 is the width of the link 14a 2 and a is the diameter of the hole 15a a coil 16a is wound around the links 14a 1 and 14a 2 a single position detector for example a hall device for detecting the position of a rotary magnet 10 relative to the armature 11 may be disposed.