When a charged body is brought to the rod of the electrometer, its arrow is deflected, since the electric field causes the separation of charges in the rod. If you cover the rod of the electrometer with a hollow metal ball and bring up the charged body again, the arrow will not deviate. Consequently, there is no electric field inside the hollow metal ball.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.