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BELL SYSTEM TECHNICAL JOURNAL 



element, a special oscillograph, which was connected to the output 

 terminals of the amplifier. The first six tubes, in cascade, provided a 

 voltage amplification of about 40,000; the last eight tubes, in parallel, 

 constituted a "current transformer" working into the low impedance 

 of the oscillograph vibrator, with a small resistance in series. The coup- 

 ling between the stages, and between amplifier and terminal apparatus, 



JX/O"' 





1000 ZOOO JOOO 4000 ^000 6000 jooo 600 gooo 



Fig. 2 — Curve A : Output of transmitter in volts per dyne per sq. cm. Curve B: 

 Phase lag of voltage behind pressure in condenser transmitter 



was entirely of resistance and capacity, with the capacity reactance 

 minimized. In all tests of the circuit the conden.ser transmitter and 

 the oscillograph vibrator remained in their fixed positions, as shown in 

 the diagram, so as not to disturb the electrical characteristics of the 

 circuit. The frequency characteristics of the amplifier in amplitude 

 and phase are shown in Fig. 3. In measuring the amplitude character- 

 istic a small electromoti\-e force was introduced in series with the 

 transmitter, in the input mesh; and in measuring the phase lead of 

 the output as a function of frequency use was made of the Alternating 

 Current Potentiometer of Wente (Jour. A. I. E. E. Dec. 1921) the 

 other details of procedure being as usual. 



The characteristics of the oscillograph \ibrator are shown in Fig. 4. 

 This vibrator was specially constructed, with small mass, high tension 

 and damping; when the requisite dynamical characteristics were once 

 obtained, its calibration presented no great difficulty. 



