“The curl of A is the circulation of the vector A per unit area, as this area tends to zero and is in the direction normal to
the area when the area is oriented such that the circulation is maximum.”
The curl of a vector field is, therefore, a vector field, defined at any point in space.
From the definition of contour integration, the normal to a surface enclosed by a contour is given by the right-hand rule as
shown in Figure 2.21 which also gives the direction of the curl. The definition in Eq. (2.87) has one drawback: It looks
hopeless as far as using it to calculate the curl of a vector. Certainly, it is not practical to calculate the circulation and then use
the limit to evaluate the curl every time a need arises. To find a simpler, more systematic way of evaluating the curl, we
observe that curl A is a vector with components in the directions of the coordinates. In the Cartesian system, for example, the
vector B ¼ curl A can be written as