ELECTRICAL MEASUREMENT 71 



tance balance, cross-talk, flutter and modulation. Since the three 

 impedance components mentioned above, together with frequency, 

 are probably of more general interest, this paper will be devoted largely 

 to a discussion of their measurement, only brief reference being made 

 to the methods used for the measurement of the latter^ group of 

 characteristics. 



As in all measurement work, standards representing the quantity 

 are required, and these are of two classes, prime standards and second- 

 ary or working standards. In our case, the prime standards are 

 resistance and frequency.. From these we derive inductance and 

 capacitance. Working standards are stable types of inductance coils, 

 air and mica condensers, adjustable resistances, and for frequency, 

 resonance type meters and highly stable oscillators. 



Prime Standards 



Frequency. The standard of frequency used is that described by 

 Horton, Ricker and Marrison.- 



Briefly, it comprises a special self-driven fork held at constant 

 temperature and having all other conditions of operation so thoroughly 

 controlled that a high degree of frequency stability is obtained. The 

 exact frequency is measured by driving synchronously a phonic wheel 

 for determining the number of cycles occurring in a given time interval. 

 This time interval is usually a period of 24 hr. as measured by time 

 signals received from Arlington. The average frequency of this fork 

 is capable of being held constant and measured in this way with an 

 accuracy of about 0.001 per cent. 



The frequency of 100 cycles obtained from this fork is used to drive 

 a 1000-cycle slave fork from which an equally constant 1000-cycle 

 frequency is obtained. Having these frequencies, all other frequency 

 measurements may be made with as high an accuracy as desired by 

 direct comparison, using the cathode-ray tube as described in detail 

 by Rasmussen.^ 



Resistance. Resistance standards specially designed for use with 

 direct currents and having a very high degree of stability may be 

 readily purchased or constructed and calibrations to a high degree of 

 accuracy may be obtained from the Bureau of Standards. These 

 resistance standards are not suitable for precision measurements at 

 high frequencies, usually being wound on metal spools, and the value 

 of the phase angle receiving only secondary consideration. It is 



^Horton, Ricker and Marrison, "Frequency Measurement in Electrical Com- 

 munication," A. I. E. E. Transactions, 1923. 



^ F. J. Rasmussen, "Frequency Measurements with the Cathode Ray Oscillo- 

 graph," A. I. E. E. Journal, January, 1927. 



