Proof that the E field inside a cylindrical resistor is constant

[Fibo112]

I am reading a proof for this statement and I don't understand one of the steps. It is stated that since the surrounding medium is nonconductive the flow of charge at the surface has no component along the normal of the surface. From this the conclusion is drawn that the E field along the normal must be zero too.

This I don't understand. Since the conductivity of the surrounding medium is assumed to approach zero couldn't the E field be nonzero without causing charge to flow?

 

[Nugatory]

if you could post a link or a citation so we know exactly what you're reading, you may get better answers.

 

[Fibo112]

if you could post a link or a citation so we know exactly what you're reading, you may get better answers.