An electrical conductor is a substance in which electrical charge carriers, usually electrons, move easily from atom to atom with the application of voltage. Conductivity, in general, is the capacity to transmit something, such as electricity or heat. Pure elemental silver is the best electrical conductor encountered in everyday life. Copper, steel, gold, aluminum, and brass are also good conductors. In electrical and electronic systems, all conductors comprise solid metals molded into wires or etched onto circuit boards. Some liquids are good electrical conductors. Mercury is an excellent example. A saturated salt-water solution acts as a fair conductor. Gases are normally poor conductors because the atoms are too far apart to allow a free exchange of electrons. However, if a sample of gas contains a significant number of ions, it can act as a fair conductor. A substance that does not conduct electricity is called an insulator or dielectric material. Common examples include most gases, porcelain, glass, plastic, and distilled water. A material that conducts fairly well, but not very well, is known as a resistor. The most common example is a combination of carbon and clay, mixed together in a specific ratio to produce a constant and predictable opposition to electric current. Substances called semiconductors?act as good conductors under some conditions and poor conductors under other conditions. Silicon, germanium, and various metal oxides are examples of semiconductor materials. In a semiconductor, both electrons and so-called holes (electron absences) act as charge carriers. At extremely low temperatures, some metals will conduct electricity better than any known substance at room temperature. This phenomenon is called superconductivity, and a substance that behaves that way is called a superconductor