All machinery and processes need some means of control to ensure proper functioning. In the past, when machinery was slow functioning, a human operator was able to keep engine speeds reasonably constant by manual changing of the valve gear; as machinery became faster and bigger this soon became impossible and automatic control had to be introduced. Early examples are the automatic valve gear on steam pumps and the centrifugal speed governor applied to steam engines. Many simple control systems are present in modern houses: for instance, the ball-cock for regulating the water level in cisterns and the thermostat in a hot-water supply or heating system. More sophisticated control systems exist in refrigerators, washing machines, radios and television sets.
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