There can only be elettric current in certain types of materials and they can be divided into three groups: conductors, insulators and semiconductors.Conductors are materials through wich elettric current can flow easily, because they have a large number of free electrons. All metals are conductors, but electricity will not flow equally easily in every metal. The best conductors are silver and gold, but copper and aluminium are normally used in electric and electronic equipment and in cable and line manufacture because they are nearly as good and of course much cheaper. Pure water conducts electricity to a small extent, while salt water and solutions of salts and acids are good conductors. Insulators are materials which have few free electrons, while the other electrons available are not easily released, since a lot of energy is needed to get them out of the atoms. The opposition that these materials offer to the realese of their electrons is called resistance. Most non-metallic solids such as plastic, rubber, glass, nylon, porcelain, etc. Are insulators.