is provided an assembly 22 for selective electromagnetic control the whole system is secured to the base plate of the machine through a single support more particularly the block 20 has sliding seats for a plurality of slider cams 24 each of which has at its rear end a flat upstanding retaining strip 24a the electromagnetic control assembly 22 has several pairs of pole pieces 32 each arranged to act on a respective one of the retaining strips 24a the control assembly 22 comprises a single permanent magnet 28 flanked by blocks 30 containing bars 32 which project to form the pole pieces on these bars electromagnetic windings 34 are provided which when excited neutralize the magnetic flux created by the permanent magnet 28 in the respective pair of pole pieces each slider cam 24 can be retained in a retracted inoperative position by the flux in the pole pieces against the action of a spring 36 guided on a rod 38 and acting to bias the slider cam 24 in the direction of arrow f36 towards the needle cylinder if the windings 34 are excited the retaining magnetic flux in the pole pieces is neutralized and the spring 36 pushes the slider cam 24 in the direction of the arrow f36 into an active position as is shown in the drawing each slider cam 24 has an edge 24b comprising a thrust profile which lies in the plane of the slider cam and which is radially inclined with respect to the needle cylinder a groove 24c and an edge 24e comprising a lowering profile which is.