I 



An Oscillograph for Ten Thousand Cycles 



By A. M. CURTIS 



EflForts to extend the frequency range of oscillographs have, for the most 

 part, been directed toward increasing the natural frequency of the vibrating 

 element, which has formed the upper limit of the useful range. This paper 

 describes a new method of attack which consists in employing a vibrator 

 strung to only a moderately high natural frequency, and in equalizing the 

 response of the string by electrical circuits both up to and beyond the fun- 

 damental resonance frequency. Employing this method of equalization, 

 a galvanometer element has been developed for the rapid record oscillograph 

 which uses strings stretched to a natural frequency of 4500 c.p.s., and equal- 

 ized to from ten to twelve thousand cycles. The paper concludes with a 

 description of such a modified rapid record oscillograph and with an oscillo- 

 gram illustrating its use. 



N the past, oscillographs have been employed over a frequency 

 range extending only to a little below the natural frequency of the 

 vibrating element, and efforts to obtain a wider range have been di- 

 rected toward raising the resonant frequency of the vibrator. In the 

 present paper there is described a new method of attack that obviates 

 many of the difficulties and restrictions previously encountered. In 

 brief it consists in equalizing the natural characteristics of the string 

 by electrical networks inserted in the circuit. One part of the network 

 equalizes for the fundamental resonance Fq, and another equalizes 

 the range above this frequency. Other factors enter to limit the upper 

 frequency obtainable, but practically flat characteristics are secured 

 up to about two and one-half times the fundamental frequency of the 

 vibrator. 



The new oscillograph arose from efforts to extend the frequency 

 range of the rapid record oscillograph (Fig. 1) already described.* 

 This instrument was of the string type, and before electrical compensa- 

 tion could be applied, a complete study of the string characteristics 

 of the galvanometer was necessary. 



If a measurement is made of the deflection of the string by an alter- 

 nating current of constant value but variable frequency, it is found 

 that the sensitivity increases enormously in the region of its funda- 

 mental resonance frequency (Fq) and that there are subsidiary reson- 

 ance peaks occurring at approximately SFq, SFq, 1 Fq, and so on. No 

 signs of resonance appear at even multiples of the fundamental fre- 



^ Electronics, August 1931, p. 70; Jour. S.M.P.E., January, 1932, p. 39; and Bell 

 Laboratories Record, August, 1930, p. 580. 



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