prevent any rebound of the piston 13 back toward the left this two stage venting or blow down provides a more gradual and more easily controlled deceleration of piston motion the main piston 13 has reached its righthand extreme in fig7 the respective annular openings 59 and 63 are venting chambers 35 and 51 to low essentially atmospheric pressure and the piston 13 is held or latched in the position shown by the intermediate pressure in chamber 37 from the intermediate pressure source openings 43 the return or leftward piston motion from the position of fig7 back to that of fig1 upon energization of coil 23 follows essentially the same sequence of events as has been described and should be clear from the symmetry of the actuator the tasks of the magnets 21 and 23 are to hold the air control valves 15 and 17 in their closed positions until neutralized by energization of the corresponding one of the coils 25 or 27 and to reclose the control valves subsequent to actuation these holding and restorative forces required of the magnets are determined primarily by the force exerted by the internal unbalanced air pressure acting on the corresponding control valve that force is in turn proportional to the projected component of valve area 29 in a plane normal to axis 12 which is exposed to unopposed high pressure air within the actuator a reduction in this effective area will result in a reduction in the required magnetic field a reduction in the size and cost of the magnets and a reduction in the required ampere turns required of the coil to neutralize that magnetic field such an area limiting feature is best understood by referring to fig9 the area reduction is made possible by reducing the valve cross sectional area.