voltage of a lightning strike, however, is many times higher than that needed to move electrons through the two air gaps. The result is that electrons go through the lightning arresters rather than traveling on to the television set and destroying it. A lightning arrester may be a spark gap or may have a block of a semiconducting material such as silicon carbide or zinc oxide. "Thyrite" was the trade name used by General Electric for the silicon carbide composite used in their arrester and varistor products. Some spark gaps are open to the air, but most modern varieties are filled with a precision gas mixture, and have a small amount of radioactive material to encourage the gas to ionize when the voltage across the gap reaches a specified level. Other designs of lightning arresters use a glow-discharge tube (essentially like a neon glow lamp) connected between the protected conductor and ground, or voltage-activated solid-state switches called varistors or MOVs. Lightning arresters used in power substations are large devices, consisting of a porcelain tube several feet long and several inches in diameter, typically filled with discs of zinc oxide. A safety port on the side of the device vents the occasional internal explosion without shattering the porcelain cylinder. Lightning arresters are rated by the peak current they can withstand, the amount of energy they can absorb, and the breakover voltage that they require to begin conduction. They are applied as part of a lightning protection system, in combination with air terminals and bonding. See also Lightning rod Surge arrester Thyristor switched capacitor Faraday cage References External links "Electrical Devices and How They Work, Part 14: Lightning Arresters" (article about lightning arresters and how they were used in early AC and DC power distribution systems), Popular Science monthly, February 1919, 5 unnumbered pages, scanned by Google Books OSHA Electric Power eTool: Illustrated Glossary: Lightning Arresters An extensive dictionary of terms used with arresters at ArresterWorks.com History of arresters at ArresterWorks.com Graphic Overview of how an arrester works NEMA Surge Protection Institute Lightning Electrical safety