with(plots): with(VectorCalculus):
exphelix2 := <exp(t)*cos(t), exp(t)*sin(t), exp(t)>;
simplify(Curvature(exphelix2, t));
simplify(Curvature(exphelix2, t),assume = positive);
simplify(Torsion(exphelix2, t));
simplify(Torsion(exphelix2, t),trig);

helixt := <cos(t), sin(t), 10*t/(2*Pi)>;
simplify(Curvature(helixt, t));
ArcLength(helixt, t = 0 .. 2*Pi);

To calculate the length of the outer annulus, we set up a proportion: the length of the inner annulus/its radius is the length of the outer annulus/its radius