Inside a conductor, the electrostatic field is zero.

Explanation:
When an external electric field is applied to a conductor, the charges within the conductor redistribute themselves in such a way that the electric field inside the conductor becomes zero. This redistribution of charges is due to the property of conductors to allow the free movement of charges.

Electric Field and Charge Distribution in a Conductor:
- Initially, when the electric field is applied to the conductor, the free charges within the conductor start to move. Electrons, being negatively charged, move towards the side of the conductor opposite to the direction of the electric field, leaving behind positive charges.
- As the electrons accumulate on one side, the positive charges accumulate on the other side, creating an electric field within the conductor that opposes the external electric field.
- This redistribution of charges continues until the electric field inside the conductor becomes zero.
- Once the electric field inside the conductor becomes zero, the charges within the conductor reach a state of electrostatic equilibrium.

Consequences of Zero Electric Field:
- Since the electric field inside the conductor is zero, there is no force on the charges within the conductor. Therefore, the charges come to rest and remain in a stable configuration.
- In electrostatic equilibrium, the charges on the surface of the conductor are uniformly distributed. The electric field just outside the conductor is perpendicular to the surface and is called the surface charge density.
- Any excess charge placed on the conductor spreads uniformly over its outer surface. This is known as the Faraday's Ice Pail Experiment.
- The electric field just outside the conductor is not affected by the presence of the conductor and is the same as it would be in the absence of the conductor.

Conclusion:
Inside a conductor, the electrostatic field is zero due to the redistribution of charges within the conductor in response to an external electric field. This property of conductors plays a crucial role in shielding the interior from external electric fields and ensuring that charges within the conductor remain in a stable configuration.