78 



BELL SYSTEM TECHNICAL JOURNAL 



driven vibrating systems, by marked variations in the electrical charac- 

 teristics that the system presents. Measurements of impedance, re- 

 sistance, and reactance of a rapid record oscillograph galvanometer 

 tuned to 2970 cycles are shown in Figs. 2, 3, and 4, respectively. 



1000 



2 



I 800 



O 



200 



2400 2500 2600 2700 2800 2900 3000 3100 3200 3300 3400 3500 3600 

 FREQUENCY IN CYCLES PER SECOND 



Fig. 3 — Variation in resistance near fundamental resonance frequency with image 

 amplitude constant at 2 mm., peak to peak. 



If approximate equalization is desired only to a frequency a little 

 below Fo, and if maximum sensitivity is not essential, it is sufficient 

 to shunt the galvanometer with a suitable resistance. Four ohms is 

 about the right value for the instrument under discussion, and gives a 

 deflection vs. frequency curve as shown in Fig. 5. There is a decided 

 peak of sensitivity at ^Fq with the result that when a current with a 



i 200 

 O 



2400 2500 



2600 2700 2800 2900 3000 3100 3200 3300 

 FREQUENCY IN CYCLES PER SECOND 



3400 3500 3600 



Fig. 4 — Variation in reactance near fundamental resonance frequency with image 

 amplitude constant at 2 mm., peak to peak. 



