Fig. 11.13. Construction of an orthonormal lead system utilizing the image surface. The lead 34 for surface points P3 - P4 is solely in the z direction, whereas lead 12 established for surface points P1 - P2 is solely in the y direction. Since 34 / 12 = (a + b)/b, the resistor network Ra and Rb is inscribed to reduce the voltage from (P3 - P4) by b/(a + b), hence making the effective z-lead equal to the y-lead in magnitude.