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BELL SYSTEM TECHNICAL JOURNAL 



With this arrangement the sensitivity falls off rapidly beyond Fo, 

 but recent advances in the art of designing equalizing networks have 

 made it possible to combine with the string already equalized to its 



O lO 



09. 



1000 2000 3000 4000 5000 6000 7000 8000 9000 10,000 11,000 12,000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 6— Characteristics of galvanometer with resonant shunt, alone for curve A, and 

 with an additional shunt to suppress the resonance at 3i^o, for curve B. 



natural frequency of vibration, a second equalizer, designed by E, L. 

 Norton, which extends the range of frequencies through which the 

 deflection is proportional to the current to a point considerably higher 

 than Fq. This is, of course, accomplished at the expense of a corres- 

 sponding reduction in sensitivity. While a variety of combinations 

 of Fo and equalizers is possible, a particular case in which a string was 

 tuned to 4500 c.p.s and equalized to 10,000 is illustrated in Fig. 7, which 

 shows the circuit of the equalizer and the characteristic obtained. 



As has already been noted, former practice has required an increase 

 of the natural frequency of the vibrator, and the employment of the 

 galvanometer only up to this frequency. Such an increase in natural 

 frequency may be obtained by increasing the tension of the string, by 

 decreasing its mass, by shortening its free length, or by a combination 

 of one or more of these modifications. There are rather severe re- 

 strictions to this method, however. Both the diameter to which the 

 wire may be drawn and the stress that may be applied are limited. 

 The string employed for both the earlier and present oscillographs, a 

 duralumin wire 0.0008 inches in diameter, approaches the best available 

 combination of mass and strength, and for the length employed, 6000 

 cycles is about at the upper limit of fundamental resonance obtainable. 



