You can show by experiment that there is no charge inside a hollow charged conductor - all the charge is on the surface. This means that the electric field inside the sphere is also zero and the potential is constant. The variation of electric field intensity and the electric potential for a hollow charged conductor is shown in the following graphs.
There will come a point where the insulating properties of an insulator break
down if the potential difference across it and hence the electric field strength within it is large
enough. This is called the
breakdown field strength.
The value of the breakdown field strength
for dry air is in the region of 5x105 Vm-
1.