The Vibratory Characteristics and Impedance of Telephone ReceiversAt Low Power Inputs

HE ordinary telephone receiver is one of the most sensitive known detectors of weak alternating currents over a considerable part of the audible frequency range. Its high sensitivity, combined with its simplicity and convenience, have led to its general adoption as the detecting element in the AC impedance bridge and other measuring apparatus employing the nul method. There are also a number of cases outside of the laboratory where a knowledge of the behavior of the receiver operating near its minimum audible power input is of importance. In apparatus developed during the World War, such as that for detecting and locating submarines, in radio reception, and in the reception of various other sorts of signals, the receiver is frequently operated near the threshold of audibility. While it is in general possible to employ a vacuum tube amplifier to render weak signals more easily audible, considerations of cost or increased complication often make it impracticable to do so. In any case, if it is desired to reduce to the limit the minimum audible signal, it is necessary to know the constants of the receiver working on these low power inputs, in order to design intelligently its circuits and other associated apparatus.