ANALYZER FOR COMPLEX ELECTRIC WAVES 231 



noting the meter deflection and referring to calibrations of the analyzer 

 which had been made with known input currents. 



This elementary form of measuring circuit was developed during the 

 World War for the analysis of the sound waves encountered in listening 

 devices used for the detection and location of submarines and torpedoes. 

 It covered the range of audible frequencies and had sufficient sensi- 

 tivity for its original purpose. 



It will be remembered that the first commercial application of 

 multiplex transmission by means of carrier currents came almost 

 simultaneously with the Armistice. The continued study of carrier 

 systems found a useful tool in the current analyzer but placed consider- 

 ably more rigorous recjuirements on its performance. These were met 

 by the addition of a second tuned circuit and amplifier system, working 

 from the output of the first, thus giving far greater selectivity than is 

 obtainable in a single circuit. The presence of the multi-stage 

 amplifier between the selective circuits facilitates tuning by avoiding 

 interactions between the circuits. A second modification was the use 

 of a substitution method for evaluating the amplitude of the selected 

 components, as with the considerable increase in the ranges of ampli- 

 tude and frequency covered, the calibration method for measuring the 

 current became impracticable. To evaluate the current, the output 

 from a sine wave oscillator, which was tuned to the same frequency as 

 the component being measured, was substituted at the input to the 

 analyzer and the amplitude adjusted until it gave the same meter 

 deflection as the unknown component. Since the current from the 

 oscillator is of the same frequency and amplitude as the original com- 

 ponent, we can determine the magnitude of the latter by measuring the 

 oscillator output. A convenient means for doing this is to interpose 

 between the oscillator and the analyzer a variable attenuator. It is 

 then possible to fix the oscillator output current at some convenient 

 value, such as 1.0 milliampere or 10 milliamperes and to adjust the 

 input to the analyzer by means of the attenuator. The current can 

 then be read directly from the attenuation tables, it being only neces- 

 sary to know the location of the decimal point. 



The development of the analyzer in this form was carried to the 

 limit of its practicability by F. Mohr. With an analyzer containing 

 three units it was possible to carry through an extensive study of the 

 modulation introduced into the Key West-Havana cable due to the 

 non-linearity of the characteristics of the iron used for loading. 



