a solenoid for an electromagnetically operated valve includes a bobbin having a substantially rectangular or elliptical cross section , a pole plate stationary with respect to the bobbin , and an armature slidable within the bobbin in response to a magnetic field generated by the coil through the pole plate . a coil wound around the bobbin has a rectangular cross section which on a short axis side includes a width w . a relation between width w and a virtual cylindrical iron core of diameter d having the same cross sectional area as an armature cross sectional area is expressed as d = w . a ratio of a length a of a long axis side of the armature to a length b of a short axis side of the armature has a range between 3 . 1 ≦≦ 4 . 5 .