The question is A long metal cylinder with radius a is supported on an insulating stand on the axis of a long, hollow, metal tube with radius b. The positive charge per unit length on the inner cylinder is λ, and there is an equal negative charge per unit length on the outer cylinder. Calculate the potential V(r) for r<a. (Hint: The net potential is the sum of the potentials due to the individual conductors.) Take V=0 at r=b. i know how to do the problem, the only question i have is what do i use for the reference in the integral for change in voltage? i know how to do spheres and all and calculating e-fields is easy, but how do i know when to use infinity as a reference and when to use something else as a reference? and what would that something else be? 2. Relevant equations Even though it's inside the first cylinder do i set the reference at infinity?