74 BELL SYSTEM TECHNICAL JOURNAL 



stancy with variations in time, current or saturation, atmospheric 

 conditions, and frequency. It is also desirable that they be made 

 with a small external field. Otherwise, very great care must be 

 taken to avoid errors due to this cause. 



In order to obtain stability with variations in saturation, it is usual 

 to make inductance standards with air cores. This requires standards 

 of large physical size if a time constant as large as the average iron 

 core coil is desirable. This large size results in large capacitance 

 distributed in the coil itself and from the coil to ground. These 

 capacitances cause large variations in inductance with frequency and 

 with the position of the coil with respect to ground. On account of 

 this difficulty with air core coils, permalloy ^ as core material has 

 been used with considerable success as described by one of the authors. '' 



The calibration of these inductance standards may be made by 

 comparison with any two of the quantities, capacitance, resistance and 

 frequency. Comparison with frequency and resistance may be made 

 in a bridge circuit exactly similar to the one used for capacitance 

 determination, substituting inductances for capacitances. A com- 

 parison with frequency and capacitance may be made by means of 

 a resonant method, and comparison with capacitance and resistance 

 may be made by means of the Owen bridge.^ The resonant method 

 is used generally except for those cases requiring large capacitance, 

 in which cases the Owen bridge is used. 



Frequency. As a secondary standard of frequency for use with 

 th cathode ray tube, where practically only one standard frequency 

 is required, a special 1000-cycle oscillator is used, designed particularly 

 for high stability of frequency with ordinary variations in external 

 conditions. This oscillator is shown in Fig. 2. It allows the use of 

 a cathode ray tube for frequency measurements with a high degree of 

 accuracy under conditions where the prime standard of frequency is 

 not accessible. 



Where a portable frequency standard is desirable, for instance, as 



a means of shop frequency checks, a resonance type of meter is used. 



This is shown in Fig. ?>. It is essentially a resonance bridge circuit 



consisting of two equal resistance ratio arms, a third arm containing 



a resonant circuit, and a variable resistance as the fourth arm. The 



capacitance and resistance are variable over wide ranges by means 



of decade switches, and the capacitance is capable of fine variations 



by the use of a form of precision variable air condenser having provision 



for fine control. There are four air-core inductance coils which give, 



6 H. D. Arnold and G. W. Elmen, Franklin Institute Journal, Vol. 195, 1923. 

 * D. Owen, "A Bridge for the Measurement of Self-Inductance," Proieedings of 

 the Physical Society of London, October, 1914. 



