Has n, f. (2017). The Effect of Air Gap Width (S) and the Ratio (P1/D2) on .Minimum Projector Focal Length (Fp)min for the Double Pole-Piece Lens- at Costant Excitation (NI). Ibn AL- Haitham Journal For Pure And Applied Science, 21(3), 95-111. Retrieved from http://jih.uobaghdad.edu.iq/index.php/j/article/view/1412