Using Gauss law, we proved that the electric field inside the charged spherical shell is zero, Further, we showed that the electric field inside both hollow and solid conductors is zero. It is a very interesting property which has an important consequence.
Consider a cavity inside the conductor as shown in Figure 1.48 (a). Whatever the charges at the surfaces and whatever the electrical disturbances outside, the electric field inside the cavity is zero. A sensitive electrical instrument which is to be protected from external electrical disturbance is kept inside this cavity. This is called electrostatic shielding.
Faraday cage is an instrument used to demonstrate this effect. It is made up of metal bars configured as shown in Figure1.48 (b). If an artificial lightning jolt is created outside, the person inside is not affected.
During lightning accompanied by a thunderstorm, it is always safer to sit inside a bus than in open ground or under a tree. The metal body of the bus provides electrostatic shielding, since the electric field inside is zero. During lightning, the charges flow through the body of the conductor to the ground with no effect on the person inside that bus.