vertex surface 45 a first upper stop insertion hole 50 that is a horizontally elongated slit is formed in the side surface of each handle like portion 49 a first upper aperture stop 35 is inserted into the first upper aperture stop insertion holes 50 the handle like portions 49 of the upper stop pushing plate 48 are connected to the spacer 20 near the top portion of the top polepiece 18 via two pairs of tension springs 51 constituting a second pushing mechanism the upper stop pushing plate 48 is pushed into intimate contact with the top polepiece vertex surface 45 by the tensile force from the tension springs 51 a pair of actuators 52 forming a third pushing mechanism utilizing a piezoelectric device for example is mounted between the handle like portions 49 of the upper stop pushing plate 48 and the spacer 20 such that almost no electric or magnetic field is produced during operation when the actuators 52 are not in operation the upper stop pushing plate 48 is kept in intimate contact with the top polepiece vertex surface 45 during operation of the actuators 52 respective one ends of the actuators 52 push down their respective handle like portions 49 bringing the upper stop pushing plate 48 out of intimate contact with the top polepiece vertex surface 45 as a result a slight gap is formed between the plate 48 and the vertex.