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BELL SYSTEM TECHNICAL JOURNAL 



tendency to pause at the top of the swing before starting downward 

 makes A easy to read, and the failure to do so explains the observed 

 "jittery" motion of instruments such as B and C. 



As a further part of this study, high speed moving pictures were 

 taken of the available volume indicators, showing their response to 

 suddenly applied sine-waves. The pictures were taken at 400 frames 

 a second and included on the edge of each frame was a photograph of 

 a clock device which indicated time in thousandths of a second. From 



i 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 I.I 1.2 1.3 



TIME (1.0 = TIME TO REACH 99 PER CENT FINAL DEFLECTION) 



Fig. 10 — Effect of damping on instrument characteristics. 



measurements made on these films, the data plotted in Fig. 11 were 

 obtained. It is interesting to observe how lightly damped are the 

 oscillations of the 203C volume indicator, which until the advent of 

 the new instrument has been in use in considerable numbers. The 

 curve for the peak volume indicator on Fig. 11 must not be mistaken 

 for the true speed of response but is merely the speed with which the 

 instrument reads the charge on the condenser (see Fig. 2). The charge 

 builds up quite rapidly, but the instrument follows in more leisurely 

 fashion as shown. The instrument, as noted earlier, will actually give 



