The process of removing small electrostatically charged particles from a gas stream, such as ash from combustion processes, using highly charged plates or tubes, and of the opposite polarity. An electrostatic precipitator is the apparatus used to bring about the separation in which the charged plates or tubes collect the particles and are cleaned periodically, either by strongly tapping the plates, or by surface cleaning with water. They are typically used to clean the flue gases from coal-fired power stations and can operate effectively at high temperatures and pressures. They are expensive to operate and can tend to be inefficient. They were first commercialized by the American physicist and inventor Frederick Gardner Cottrell (1877–1948) in 1907.