The Vibratory Characteristics and Impedance of 

 Telephone Receivers at Low Power Inputs 



By A. S. CURTIS 



THK <)r(linar>- telephone recei\er is one of the most sensitive 

 known detectors of weak alternatinj; currents over a considerable 

 part of the audible frcquenc\- range. Its high sensitivity, combined 

 with its simplicity and convenience, have led to its general adoption 

 as the detecting element in the A(" 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 recei\er o[)erating 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 audibilit>'. 

 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 intelligenth' its circuits 

 and other associated apparatus. 



Current literature dealing with the sensitivity of telephone receivers 

 indicates that the relation between the impedance and \ibratory 

 characteristics of the receiver at currents near minimum audibility to 

 those as ordinarih- determined in the laboratory, is not generally 

 known. It would, therefore, seem of interest to publish the results 

 of an experimental determination of receiver characteristics at very 

 low currents. Such an investigation was carried on in 1918 and 

 1919, using the Western Klectric Xo. 509 radio receiver (the present 

 standard Western Electric Receiver for radio use). The work, 

 however, was done, not merelj' with the idea of determining the 

 characteristics of this particular instrument, but for the purpose of 

 ascertaining the behavior of receivers in general, near minimum 

 audibility. 



Inasmuch as the daiiipid impidance of tin- receiver- that is liie 

 impedance with the dia|>hragm held motionless — is very close to 

 the impedance obtained with the instrument on the ear, it is com- 

 monly used as the basis of circuit calculations. A knowledge of its 

 value for weak currents is therefore of importance. Measurements 



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