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high-gain pneumatic amplifier, which had been invented in 1914, with negative feedback from the controller output. This dramatically increased the linear range of operation of the nozzle and flapper amplifier, and integral control could also be added by the use of a precision bleed valve and a bellows generating the integral term. The result was the "Stabilog" controller, which gave both proportional and integral functions using feedback bellows. The integral term was called Reset. Later, the derivative term was added by a further bellows and

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