where formula_126 is a constant that represents the potential added by the ionic atmosphere. No justification for formula_126 being a constant is given. However, one can see that this is the case by considering that any spherical static charge distribution is subject to the mathematics of the shell theorem. The shell theorem says that no force is exerted on charged particles inside a sphere (of arbitrary charge). Since the ion atmosphere is assumed to be (time-averaged) spherically symmetric, with charge varying as a function of radius formula_128, it may be represented as an infinite series of concentric charge shells. Therefore, inside the radius formula_122, the ion atmosphere exerts no force. If the force is zero, then the potential is a constant (by definition).