fastening nut 90 a part cylindrical cowell 92 defines with the lug 80 a cylindrical chamber 94 for accommodating the gear wheel 88 the chamber 92 is open at its upper region with the apparatus viewed as shown in fig5 so that the gear wheel 88 can mesh with an externally toothed annular ring 94 which is rotatably mounted on the cap 38 is disposed coaxially with the lamp 30 and core 48 and provides the first magnetic coupling member of a magnetic linkage through which the motor 82 can rotate a rack and helical member assembly 96 around the lamp 30 to that end the annular ring 94 contains a number of radially positioned permanent magnets not shown which are equi angularly arranged within the ring with alternating polarities thus for example one of those magnets positioned with the north pole as its radially inner pole will be flanked by two other magnets in each of which the south pole will be radially innermost the second coupling member of the magnetic linkage comprises an inner annular ring 98 which contains a similar arrangement of magnets not shown and is mounted within the housing 32 in use the operation of the motor 82 rotates the gear wheel 88 which in turn rotates the ring 94 this in turn causes a corresponding rotation of the ring 98 by virtue of the.