**Magnetostatics**from

**Introduction to Electrodynamics**by

**David J. Griffiths,**we are calculating the magnetic field that an ideal toroid would create. Here are the images from the book

My doubt is how he got $\mathbf I = \langle I_s \cos \phi ', I_s \sin \phi ', I_z \rangle$. Well, I know how to convert from cylindrical coordinates to cartesian coordinates but my problem is how he got the angle $\phi ' $? Why the current would make an angle of $\phi '$ with the $x-$ axis? As far as I understand the things it is the location of current, that is the vector $\vec{r'}$ that makes the angle $\phi '$ with the $x-$ axis, the current at $\vec{r'}$ have no $\phi$ component it just have a radial component ($I_s$) and a z-directional component ($I_z$) means the current vector looks something like this

Again, to restate my problem I would say : How Mr.Griffiths knew that

*current vector*would make an angle of $\phi '$ with the x axis?