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The purpose of a servo is to reproduce a signal at a place or power level or form different from the original signal, but under its control. It is therefore a signal-transmitting system. It uses negative feedback to minimize noise and distortion, which the servo designer usually calls error. It uses mechanical and thermal circuit elements as well as electrical circuit elements, but the problems of circuit design are the same. The methods of Nyquist and Bode, which have proved so useful in the design of electrical feedback amplifiers, are equally useful in the design of servo systems. They encourage the determination of the significant constants of the system by experimental means involving steady-state amplitude measurements.