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The application of feedback to an entire amplifier rather than just to the final stage makes it possible to realize the characteristics of a perfect amplifier over wide frequency ranges. The use of such amplifiers to give direct-reading audio-frequency voltmeters with permanent calibration and any desired sensitivity is described. Negative feedback can be used to reduce the distortion in the output of laboratory oscillators for all loads from open circuit to short circuit by the expedient of throwing away a part of the output power in a resistive network. Means are described for applying feedback to tuned radio-frequency amplifiers so that the amplification depends only upon the constants of the tuned circuit and is independent of the tubes and supply voltages. The use of negative feedback to develop a stabilized negative resistance substantially independent of tubes and supply voltages is considered, and various applications described. High selectivity can be obtained by deriving the feedback voltage from the neutral arm of a bridge, one leg of which involves a parallel resonant circuit. It is possible by this means to obtain an effective circuit Q of several thousand, using ordinary tuned circuits, and the selectivity can be varied without affecting the amplification at resonance. The use of these highly selective circuits in wave analyzers is considered. Feedback can be used to give improved laboratory oscillators.