Why are scientific theories modified, but seldom discarded?

A scientific theory is a well-explained idea which is established after performing the experiments which provide the evidence in support of the hypothesis.

The theory can be modified as well as can be rejected with time as the advancement in the technology can provide new facts which could be added in the existed theory or contradictory facts which could lead to the rejection of the theory.  

The generation of new data can shift the paradigm of science on the basis of which the scientific theory is based.

electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia

there are no atoms