MEASUREMENTS AT TELEPHONIC FREQUENCIES. 103 



by the observations, which, shown as round dots, lie close to the 

 circle OABD, except for an accidental wide unobserved gap between 

 0.019 and 0.022 microfarad. In this case, the resistance of the cir- 

 cuit was 250 ohms, and this relatively small resistance involves a 

 relatively large change in the vector impedance near resonance, com- 



FiG. 4. Current-locus for conditions similar to those of Fig. 2, but ap- 

 plying to a circuit of smaller resistance, and to an impressed frequency 



of 2010'—'. 



puted at 0.0214 microfarad, with a small change of capacitance. It 

 may be noted that in this case a change of 250 ohms in the condenser 

 reactance changes the vector current, from resonance, through 45° 

 difference of phase, or over a quarter of the circular locus. As the 

 resistance R in the circuit is reduced, a correspondingly smaller 

 change in reactance, from the resonant point, will produce this 45° 

 change in the vector current. An a.-c. potentiometer can thus 

 serve to measure a small capacitance with precision, by observing 



