B.4 For wires along toroidal direction

For a curve along the toroidal direction, the line element dl in terms of the cylindrical basis is written as dl = Rdϕˆeϕ. Using Cartesian coordinate basis vectors, dl is written as

dl = Rdϕ (− sin ϕˆex + cosϕˆey).
(634)

Using Eq. (634) and (626), we obtain

             ||   ˆex       ˆey      ˆez  ||
dl× (r− r′) = ||− Rd ϕsin ϕ Rd ϕcosϕ 0  ||
             || (x− x′)  (y− y′) (z − z′)||
(635)