In a metal, many electrons are easily movable. When an electric field is applied, they migrate to the surface of the object, which is why they are called free charges. The displacement continues until the electric field lines stand perpendicular to the metal surface. This phenomenon is called electrostatic induction. Since the electric field lines now originate from the metal surface, the interior of the object must be free of fields. In technology, this effect is used in the form of Faraday cages to protect sensitive electronics from electrical damage. Because airplanes and cars meet the conditions of a Faraday cage quite well, people inside them are well protected in the event of a lightning strike.
Experiment: Electroscrope
An electroscope consists of a pivoted metal pointer enclosed in a stable metal housing and electrically connected to it. If a charged glass rod is brought close to the upper electrode, the electrons inside the electroscope are displaced. As a result, both the pointer and the holder become positively charged and repel each other, causing the pointer to deflect more strongly depending on the amount of charge.