in the slot of the fixed cam 26 and simultaneously the start of the levelling cam 29 engages behind the bent arm 25e of the selection jack the sloping portion 29 39 of this cam causes the jack 25 to rock outwardly when this rocking movement is finished the preparation ramp 26 39 of the fixed cam 26 causes jack 25 to descend slightly so as to carry its upper butt 25c to a level lower than that of the selector member 27 butt 25c is then positioned under portion 27 39 of the selector member and simultaneously jack 25 is lifted by the cocking ramp 26a of cam 26 this movement of the jack 25 cocks the selector member 27 by raising it upwards and bringing it into contact with the armature of the electromagnet 28 at that time the programmed control device sends a pulse or refrains from sending a pulse according to whether the needle 23 should knit or not knit let it be supposed that the electromagnet receives a pulse the selector member remains attracted to the armature of the electromagnet once the butt 25c has left portion 27 39 of the selector member the uncocking ramp 26c of cam 26 lowers jack 25 slightly while butt 25c is in portion 27 34 of the selector preceding cam portion 27 39 34 since the selector member 27 is still attracted to the armature of electromagnet 28 butt 25c passes under cam 27 39 34 so that the selection jack remains in the position set by the levelling cam 29 jack 24 remains in the position shown by the dot and dash line in fig1 so that the head 24b of the jack.