Electric field strength and potential in a charged conducting sphere

The electric field strength inside a conducting charged sphere is zero, but why? In the book it says "that the field lines would link charges of opposite sign in the sphere and such a state of affairs is impossible under static conditions in a conductor." I don't really get this sentence.
And if the electric field strength inside a conducting charged sphere is zero, does it mean that the potential inside it is also zero? Or the electric potential is same as the potential on the surface of the sphere?
 If a field line linked two opposing charges, they would accelerate towards one another because they are free to move in a conductor. This acceleration contradicts the assumption that the conditions are static. Since the electric field is minus the gradient of the electric potential, if there is no electric field then there can be no gradient, ie. the electric potential is uniform over the conducting volume, although not necessarily zero.
 thanks :)