from a power source to the electromagnet 35 37 is a permanent magnet 38 is a rock 39 is a spacer 40 is a yoke 41 is a printed circuit board and 42 is a clamp the clamp 42 presses and holds the base 34 the permanent magnet 37 the rack 38 the spacer 39 the plate springs 33 the yoke 40 the printed circuit board 41 and the guide frame 31 in a manner such that these members are assembled one on the other in turn in such order and are integrated each armature 32 is supported at a free end 33a of a respective plate spring 33 while a base end 30a of a respective one of the print wires 30 is fixedly mounted on a distal end 32a of the armature 32 a distal end 30b of the print wire 30 is guided by the frame groove 31a of the guide frame 31 so as to strike against a predetermined position of the printing paper not shown fig9 is a plan view of the printed circuit board 41 and fig1 is a perspective view of a main portion of the printed circuit board 41 the printed circuit board 41 includes sensor electrodes 10a which are composed of copper foil patterns positioned opposite the armatures 32 and connected to connector terminals 41a of the printed circuit 41 the printed circuit board 41 is coated by an insulting film to provide insulation from the yoke 40 for thereby generating electrostatic capacitance in the interval between the sensor electrodes 10a and the armatures 32 the larger the intervals between the sensor electrodes 10a and the armatures.