**Fig. 12.20.** The isosensitivity lines for the bipolar lead of Figure 12.19. The coil radii are 10 mm and the distance between the coils is 340 mm. The vertical axis indicates the distance *h* from the first magnetometer and the horizontal axis the radial distance *r* from the symmetry axis. The symmetry axis, drawn with thick dashed line, is the *zero sensitivity line*. Lead field current flow lines encircle the symmetry axis and are illustrated with thin solid lines.