#

The circular loop of radius a lies in the x-y plane, pivoted along the z-axis, and centered at the origin. The loop carries a current element d I. A distance vector upper R joins a point P at (0, 0, z) from the differential vector d I and makes and an angle theta. The differential element produces a differential magnetic field d H, and the direction of d H is perpendicular to the plane containing upper R and d I. The d H is in the lower r-z plane, and therefore it has components d H sub lower r and d H sub z. An element d l prime, located diametrically opposite to d I. Therefore, the d I prime produces a differential magnetic field d H prime. The components of the d H prime are d H prime sub lower r and d H prime sub z. The radius of the loop makes an angle phi with the x axis counterclockwise and the line, which connects the center and another element d I prime makes an angle phi plus pi with the x-axis counter clockwise.