In case of certain materials like glass, the electrons are firmly 'bound' to the atoms and they do not normally move. Such materials are known as the 'insulators' or bad conductors of electricity. Polythene, ebonite, silk, fur, plastic etc. belong to this category. The charge produced by rubbing a glass does not flow away, because it is an insulator. These charges are called static because they do not move.
In case of metals, however, one of more electrons are weakly bound to the atom. Such electrons can easily wander from one atom to the next one in the metal. Hence, they are called 'free' electrons. Normally, the drift of free electrons is irregular, i.e., it occurs in all directions. The presence of these free drifting electrons makes the metals good conductors of electricity. If a metal rod is held in the hand and rubbed with a duster, no charge is produced on the metal. Any electrons produced by rubbing can move along the metal and pass through the body to the earth. This is because our body and the metals are conductors of electricity. A metal rod provided with a glass or ebonite handle can be electrified by rubbing with flannel. What is commonly known as an 'electric current' is nothing but a one way and orderly drift of these electrons.
