reason uncontrolled electrical arcs in apparatus become so destructive since once initiated, an arc will draw more and more current from a fixed-voltage supply until the apparatus is destroyed. Uses Industrially, electric arcs are used for welding, plasma cutting, for electrical discharge machining, as an arc lamp in movie projectors, and spotlights in stage lighting. Electric arc furnaces are used to produce steel and other substances. Calcium carbide is made in this way as it requires a large amount of energy to promote an endothermic reaction (at temperatures of 2500 °C). Carbon arc lights were the first electric lights. They were used for street lights in the 19th century and for specialized applications such as searchlights until World War II. Today, low-pressure electric arcs are used in many applications. For example, fluorescent tubes, mercury, sodium, and metal halide lamps are used for lighting; xenon arc lamps are used for movie projectors and theatrical spotlights. Formation of an intense electric arc, similar to a small-scale arc flash, is the foundation of exploding-bridgewire detonators. Electric arcs have been studied for electric propulsion of spacecraft. They are used in the laboratory for spectroscopy to create spectral emissions by intense heating of a sample of matter. Protection of electrical equipment Arc is still is being used in high voltage switchgear for protection of extra high voltage transmission networks. To protect a unit (e. g., a series capacitor in a transmission line) against overvoltage, an arc-inducing device, so called spark gap, is connected in parallel to the unit. Once the voltage reaches the air-breakdown threshold, an arc ignites across the spark plug and short-circuits the terminals of the unit, thus protecting the latter from the overvoltage. For the reinsertion of a unit, the arc needs to be extinguished, this can be achieved in multiple ways. For example, a spark gap can be fitted with a "Jacob's ladder" − two wires, approximately vertical but gradually diverging from each other towards the top in a narrow V shape, named for the Jacob's Ladder leading to heaven as described in the Bible. Once ignited, the arc will move upwards along the wires and will break down when the distance between the wires will become too large. If the arc is extinguished and the original trigger condition no longer exists (a fault has been resolved or a bypass switch engaged), the arc will not reignite. The arc can be