6. (7%, 本題計算過程須計分) Two spherical conductors of radii r₁ and r₂ are separated by a distance much greater than the radius of either sphere. The spheres are connected by a conducting wire as shown in Fig. 4. The charges on the spheres in equilibrium are q₁ and q₂, respectively, and they are uniformly charged. Find the ratio of the magnitude of the electric field at the surface of the sphere 1 to the one of the sphere 2 is      (6a)      (3%). Please use the meaning of the ratio to explain the principle of a lightning rod (避雷計). Your explanation:      (6b)      (4%)