A pilot light is a small gas flame, usually natural gas or liquefied petroleum gas, which serves as an ignition source for a more powerful gas burner. Originally a pilot light was kept permanently alight, but this wastes gas. Now it is more common to light a burner electrically, but gas pilot lights are still used when a high energy ignition source is necessary, as in when lighting a large burner. A United States patent was filed May 13, 1922 for a "safety gas-control system" by two employees of the Newark, NJ-based Public Service Gas Company, Conrad Shuck, Jr. and George Layer. The term "pilot light" is also used occasionally for an electrical indicator light that illuminates to show that electrical power is available, or that an electrical device is operating. Such indicators were originally incandescent lamps or neon lamps, but now are usually LEDs. Uses Common applications include household water heaters, central heating systems, fireplaces, flamethrowers, and hot air balloons. While most commercial kitchens still rely on pilot lights for burners, ovens, and grills, current residential systems utilize an electrical ignition. This is more commonly known as standby on modern remote control fires. Safety protection In natural gas furnaces, water heaters, and room heating systems, a safety cut-off switch is normally included so that the gas supply to the pilot and heating system is shut off by an electrically operated valve if the pilot light goes out. This cut-off switch usually detects the pilot light in one of several ways: A flame rectification device. A sensor filled with mercury is used to detect the heat of the pilot light. Contraction of the mercury results in sufficient pressure to operate an electrical switch that interrupts the flow of electricity and shuts off the gas valve when the pilot light goes out. A photoresistor is used to detect the light from the pilot lamp. When the pilot light goes out, electrical circuitry connected to the photoresistor shuts off the gas valve. Use of a pilot generator or a thermocouple in the flame provides heating appliance safety as it generates enough electric current from the burning flame to hold the gas valve open. If the pilot light goes out, the pilot generator cools off and the current stops, closing the gas valve. Other units use a non-electrical approach, where the pilot heats a bimetallic element or a gas-filled tube to exert mechanical