I'm looking to derive the equation for torsion for a space curve $\displaystyle \alpha (u)$

$\displaystyle \tau= \frac{(\alpha ' \times \alpha '')\cdot \alpha '''}{{||\alpha '\times \alpha ''||}^{2}}$

I think I use $\displaystyle b = t \times n $ and of course $\displaystyle t = \frac{\alpha '}{||\alpha '||}$

Then $\displaystyle \frac{db}{ds}=\frac{1}{||\alpha '||}\cdot\frac{db}{du}=\tau n$

But I can't see where the $\displaystyle \alpha '''$ comes in...