An electrical insulator resists the flow of electricity. Application of a voltage difference across a good insulator results in negligible electrical current. In comparison, a conductor allows current to flow readily. Controlling the flow of current in electrical wiring and electronic circuits requires both insulators and conductors. For example, wires typically consist of a current-carrying metallic core sheathed in an insulating coating.
Resistivity is the measure of a material's effectiveness in resisting current flow.
16 ohm-m can be achieved in exceptional insulating materials. The enormous variation in room-temperature resistivity is one of the largest for any physical attribute of matter. The charge carriers responsible for current in most conductors are electrons, moving relatively freely in a metal. In insulators, electrons cannot move freely. When atoms of simple metals combine to form a solid, the outer valence electrons become free for conduction.