Another way to go would to be to use Rochrs idea but using metaballs (or spheres) rather than multireplicated along a path As you can see in this case if the sprial unwinds the metaball spacing gets larger and the effect becomes less convincing. (I just used the default bryce spiral path for my rendered example so the spiral isn't right but you get the idea from this anyway.)

using Agent Smiths idea you might make a spiral with an outie convex rail and neg bool it with an innie concave rail. The hard part for me would be figuring out how to keep the gradient of the rail constant.