Abstract: A tow of filamentary dielectric material, wrapped in a flexible conveyor belt of low permittivity material such as PTFE, is passed through a stationary tube also of low permittivity material. This tube is disposed in resonating cavities wherein a high frequency electric field of 3.10.sup.8 to 3.10.sup.11 Hz heats the tow. After cooling, the tow forms a tube which may be cut to form cigarette filters.

Abstract: Faults in a composite filter rod being supplied to a sectioning device are detected optically by traversing the rod with a beam of light supplied along a first fiber bundle and taken up and delivered to photo-detector by a second fiber bundle. The ends of the cylindrical fiber bundles are held in the configuration of narrow homothetic rectangles transverse to the direction of feed of the rod, the width of the first bundle being approximately double that of the second bundle. The photo-detector controls elimination of faulty tips after sectioning; additional pairs of fiber bundles can be used to control synchronization of cutting with feed of the rod, and the correct succession of filter elements in the rod.