11.5 Assignments 359(b) Cut the ribbon all the way along its length so that it becomes two interlinked
ribbons of unequal width. Assuming that the original ribbon had afluxΦ,calcu-
late the helicity of the configuration taking into account the helicity due to both
twists and linkages and show that helicity is conserved.
(c) Demonstrate that the sum of twist plus writhe is conserved using a bundle of
flexible wires (or ropes) that are initially twisted about a straight axis and then
manipulated into a kink where the wires are parallel to the locus of the kink.The
ends of the bundle should be fixed. Similarly, start with a bundle of untwisted
wires that are parallel to a straight axis and manipulate the wiresinto a helix.
Show that the sense of the twist of the wires about their local axis is oppositeto
the sense of the helix so that a condition of zero helicity is maintained.