Two highly conducting spheres with radii a₁ and a₂ are used to inject and extract current from points deep inside a tank of weakly conducting fluid. Show that the resistance between the spheres depends very weakly on their separation d when d is large compared to a₁ and a₂. Confirm that the dependence on d disappears entirely if d is large enough. Assume that the electrolyte has permittivity and conductivity σ.