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The conductor carries a current I along the z-axis. The two points P sub 1 (z sub 1) and P sub 2 (z sub 2) are highlighted on the conductor. A point P equals (r, phi, z) is highlighted at a distance r from the conductor. The line, which connects the point P sub 2 and P, makes an exterior angle theta sub 2 with the z-axis. The line, which connects the point P sub 1 and P, makes an interior angle theta sub 1 with the z-axis.