I'm learning vector analysis at the moment and having a bit of trouble grasping what's being put in front of me. My book (Theoretical Physics by Joos) explains it as such: I understand that dt/ds is perpendicular to t, but the problem I have is where Joos says matter-of-factly that |dt| = [tex]d\phi[/tex]. I simply cannot see how he is coming by that result, geometrically or algebraically. Can anyone point me in the right direction? What am I missing?