Two long concentric cylindrical conductors of radii a and b (b < a) are maintained at a potential difference V and carry equal and opposite current I. Show that an electron with a particular velocity u parallel to the axis may travel undeviated in the evacuated region between the conductors, and calculate u when a = 50 mm, b = 2.0 mm, V = 50 and I = 100 A. It is also possible for the electron to travel in a helical path. By regarding such a path as the combination of a circular motion perpendicular to the axis with a steady velocity parallel to the axis, indicate without detailed mathematics how this comes about.