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The solenoid has length l and radius a, and comprises N turns carrying current I. The turns are in the shape of circular loops. The circular loops with dots at center represent the outward direction of the current. The circular loops with crosses at center represent the inward direction of the current. A vector represents the magnetic flux density B at the point P on the axis of the solenoid along the z-axis. The increment length d z represents the equivalent loop. The angle between the line which connects the one end of the solenoid to the point P and the x-axis is theta sub 2. The angle between the line which connects the other end of the solenoid to the point P and the x-axis is theta sub 1. The angle between the lines which connect a point at a distance z from the x-axis makes an angle theta with the x-axis. The differential angle d theta is the angle of the differential length d z.