How Griffiths has got the components of the current?

Sep 2019
22
2
I'm leaning 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

mymath.png

mymath2.png

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

PhysicsHelp.png


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?
 
Sep 2019
22
2
Can we discuss about what’s happening above? Sometimes, the complete answer is not possible, there’s I request you all to at least discuss things with me.