number 1 in fig1 indicates as a whole a conveyor ( shown partly ) for feeding a train 2 of carriages 3 along a path p defined by two rails 4 ( only one shown in fig1 ) and comprising a horizontal , straight forward portion p 1 and a horizontal , straight return portion p 2 ( both shown partly in fig1 ) parallel to each other and connected to each other by a semicircular portion p 3 extending about a horizontal central axis 5 perpendicular to the fig1 plane . train 2 comprises three identical carriages 3 connected mechanically to one another by respective connecting arms 6 , each of which is hinged to the ends of two respective adjacent carriages 3 to enable carriages 3 to rotate with respect to each other along the curved portions of path p . along straight portions p 1 and p 2 , carriages 3 are run along rails 4 by a known actuating device ( not shown ), which , in one embodiment not shown , comprises a linear electric motor , and , in an alternative embodiment not shown , comprises a releasable connecting member for mechanically connecting each carriage 3 to a chain traveling parallel to rails 4 . conveyor 1 comprises a timing device 7 for feeding train 2 of carriages 3 along portion p 3 with a given law of motion and in time with known external operating means ( not shown ) cooperating with carriages 3 along portion p 3 . as shown in fig1 and 2 , each carriage 3 comprises a frame 8 made of nonmagnetic material ( e . g . aluminium or plastic ) and supporting an upper box 9 for housing an article 10 ( defined in the fig1 example by a group of cigarettes ) to be conveyed along path p ; two horizontal pins 11 , each end of each of which extends outwards of frame 8 and defines a slide engaging in transversely sliding manner an axial seat 12 in a respective rail 4 ; two permanent magnets 13 forming part of timing device 7 and located a given distance d apart ; and a bottom pin 14 extending downwards from and integral with frame 8 . as shown in fig2 in addition to permanent magnets 13 of each carriage 3 , timing device 7 also comprises a powered shaft 15 coaxial with axis 5 and supported by a hollow fixed frame 16 via the interposition of two bearings 17 . along portion p 3 , frame 16 supports rails 4 by means of annular bodies 18 connected to rails 4 and to frame 16 by means of respective screws . timing device 7 also comprises two disks 19 fitted to shaft 15 and each supporting a number of permanent magnets 20 , each facing a permanent magnet 20 on the other disk 19 . magnets 20 of each disk 19 are divided into pairs 21 equally spaced along disk 19 , with the magnets 20 in each pair 21 separated by a distance equal to distance d . disks 19 have respective axes 22 lying in plane ii — ii , at an angle to each other , and oppositely inclined to define , between disks 19 , a variable - section , semicircular passage 23 through which travels the bottom portion of each carriage 3 supporting permanent magnets 13 . disks 19 being oppositely inclined with respect to shaft 15 , passage 23 is relatively narrow at an initial portion of portion p 3 , and relatively wide at an end portion of portion p 3 . a fixed semiannular body 24 is housed in the gap between the two disks 19 and radially inwards of passage 23 , and has , along the outer periphery , a semicircular groove 25 coaxial with shaft 15 and engaged in transversely sliding manner by bottom pins 14 of carriages 3 . shielding members 26 made of ferromagnetic material ( preferably magnetic steel ) are fitted in fixed positions to body 24 and to rails 4 so as to be interposed between permanent magnets 20 of disks 19 and permanent magnets 13 of carriages 3 at an initial portion and end portion of portion p 3 ; and the thickness of shielding members 26 varies from a maximum at the ends facing portions p 1 and p 2 , to a minimum of substantially zero at the ends facing the central portion of portion p 3 . thrust bearings 27 are interposed between frame 16 and intermediate portions of disks 19 to enable the axial thrust produced by in - service electromagnetic interaction of permanent magnets 13 and 20 to be transmitted by disks 19 to frame 16 . operation of timing device 7 will now be described with reference to fig1 and 2 and to a train 2 comprising three carriages 3 as in the example shown . train 2 is fed along portion pi by said known actuating device ( not shown )— preferably a linear electric motor — until the first carriage 3 in train 2 engages portion p 3 , where the bottom portion of carriage 3 engages passage 23 , and the interacting magnetic forces established between the two permanent magnets 13 of carriage 3 and two corresponding pairs 21 of permanent magnets 20 on the two disks 19 move and lock carriage 3 into a given fixed position with respect to disks 19 . from this moment on , train 2 is fed along portion p 3 in perfect time with the rotation of disks 19 , by each pair 21 of magnets 20 acting as a timing member to time carriages 3 with disks 19 along portion p 3 . for timing device 7 to work properly , the polarities of a permanent magnet 13 and of the two respective permanent magnets 20 facing magnet 13 in use must obviously be opposite at all times , so that the forces between magnets 13 and 20 are always of attraction and never of repulsion . this condition is met substantially at all times , by the positioning error of train 2 of carriages 3 with respect to disks 19 being relatively small ( about a few millimeters ) at all times , and such as never to result in a relative position in which opposite polarities are positioned facing . magnets 13 are maintained in a substantially fixed position with respect to respective magnets 20 along portion p 3 , but change position with respect to magnets 20 along the portions connecting portion p 3 to portions p 1 , p 2 ; which change in position is opposed or assisted by the forces of magnetic attraction generated along portion p 3 , thus resulting in severe mechanical stress of both carriages 3 and disks 19 . the purpose of shielding members 26 is to considerably reduce the intensity of such stress , which is done by interposing a member of ferromagnetic material between a magnet 13 and respective magnet 20 , so that the magnetic field lines of each magnet 13 , 20 tend to close in the iron without affecting the other magnet 20 , 13 , thus greatly reducing the forces of electromagnetic attraction between the two magnets 13 and 20 . the tilt of disks 19 with respect to shaft 15 provides for obtaining a passage 23 varying in width along portion p 3 , and , hence , a force of electromagnetic attraction between magnets 13 and 20 which also varies along portion p 3 by being closely dependent on the distance between magnets 13 and 20 . for effective timing , while at the same time reducing mechanical stress , disks 19 are so tilted as to obtain a relatively narrow passage 23 where more accurate timing is required , and a relatively wide passage 23 where magnets 13 and 20 move away from one another . in the example shown , the choice made is for extremely precise position control at an input portion of portion p 3 . in an alternative embodiment not shown , permanent magnets 13 are replaced by members made of ferromagnetic material and which , in use , interact with the magnetic field generated by permanent magnets 20 . in yet a further embodiment not shown , permanent magnets 20 are replaced by electromagnets . timing device 7 as described is particularly advantage for use in conjunction with said known actuating device ( not shown ) comprising a linear electric motor , by permanent magnets 13 of carriages 3 being used as part of both timing device 7 and the linear electric motor .