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A mathematical analysis of the effect which fluctuation voltages have on the operation of a linear detector is presented. Expressions for the audio-frequency noise spectra and the root-mean-square values of the audio noise outputs under various conditions of operation are derived. The demodulation and modulation-compression effects which fluctuation voltages introduce in the useful signal output are evaluated. The harmonic distortion of the useful signal output caused by the presence of fluctuation voltages is given. Finally, an expression for the audio signal-to-noise ratio which results when fluctuation voltages are applied simultaneously with a modulated carrier is derived. Experimental equipment and methods used for checking many of these results are described and experimental results are presented.