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BELL SYSTEM TECHNICAL JOURNAL 



sistance which can be varied, and a radio frequency ammeter. An 

 oscillator is coupled loosely to the inductance and its frequency varied 

 until resonance is obtained as indicated by maximum current through 

 the meter. Without changing the tuning, the resistance is changed 



Oscillator g 



C C 



iV) WVWvW 



^G 



Fig. 1 



and a second reading of current is obtained. Then, since the e.m.f. 

 induced in the measuring circuit is the same in both cases, if Ri and 

 Ri are the known resistances, I\ and I2 the corresponding currents, 

 and r the resistance of the remainder of the circuit, 



h (r + R,) = h {r + R,), 



R2I2 — Rili 



/i 



or, if Ri be made zero 



r = 



R, 



A standardized variable air condenser having negligible resistance 

 is then substituted for the condenser under test and the process re- 

 peated except that the circuit is tuned to resonance by varying the 

 capacity instead of the frequency. In this way the resistance of the 

 circuit exclusive of the test condenser may be determined. The 

 difference between these two circuit resistances is the resistance of 

 the test condenser from which the phase difference may be computed. 

 The capacity of the test condenser is equal to the capacity of the 

 standard condenser which produces resonance. From this and the 

 dimensions of the sample, its dielectric constant may be computed. 



In addition to the general precautions mentioned in the Bureau of 

 Standards Bulletin, two others should be observed in the measure- 

 ment of dielectrics. First, the electrodes must be in intimate con- 

 tact at all points with the surface of the sample, as a very small air 

 space will cause a large error in the values of phase difference and di- 



I 



