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The inner sphere has a higher potential than outer if the charge q is positive.
If the two spheres are connected through a wire, the charge will flow from high to low potential.
Thus, providing smaller potentials at the inner sphere will keep building large amount of charge at the outer sphere, till the breakdown field of air( 3 x 10 6 V / m) is reached.
o This accumulates close to millions of volts.
Construction