In vector calculus, the curl (or rotor) is a vector operator that describes the rotation of a vector field. At every
point in the field, the curl is represented by a vector. The attributes of this vector (length and direction) characterize
the rotation at that point.
The direction of the curl is the axis of rotation, as determined by the righthand rule, and the magnitude of
the curl is the magnitude of rotation. If the vector field represents the flow velocity of a moving fluid, then
the curl is the circulation density of the fluid. A vector field whose curl is zero is called irrational. The curl
is a form of differentiation for vector fields. The corresponding form of the fundamental theorem of calculus is
Stokes' theorem, which relates the surface integral of the curl of a vector field to the line integral of the vector
field around the boundary curve.  Wikipedia
"Ñ" is a vector and is pronounced "del." The vector function is A(x,y,z),
A(r,Φ,z), or A(r,θ,Φ)
Cartesian Coordinates 

Cylindrical Coordinates 

Spherical Coordinates 

