A conductor conducts electricity, and an insulator, or a dielectric, does not. A semiconductor falls between the two extremes and conducts less electricity than a conductor but more than an insulator. Siemens per meter (S/m) and reciprocal seconds are measures of conductivity. It is represented in equations by the Greek letters sigma, kappa or gamma.
Conductors offer little resistance to electric currents, which allow charges to flow through them easily. Most metals are good conductors; the atoms of metals contain loosely bound electrons that are free to move and form electric currents. Electrolytes are liquids that allow the flow of positive and negative ions, and superconductors offer no resistance to electric current under certain conditions. Insulators such as rubber, glass and porcelain offer a great deal of resistance to the flow of electricity.
Covering a conductor with an insulating material, such as encasing wire with rubber or plastic, restricts electric current to the conductor and prevents charges from escaping into surrounding materials. Semiconductors including silicon, selenium and cadmium are important in the manufacture of transistors, rectifiers and the components of integrated circuits used in electronic equipment. The conductivity of a semiconductor varies under different conditions; it changes depending on the composition of the semiconductor material and exposure to light, electric fields and heat.