72 BELL SYSTEM TECHNICAL JOURNAL 



necessary, therefore, to use resistance standards of special construction, 

 depending upon the particular application to be made. In all cases, 

 constancy of resistance with variations in atmospheric conditions, 

 frequency and time is imperative. Generally as small a phase angle 

 as possible is also highly desirable, although for some uses a suitable 

 degree of constancy may be sufficient provided that the angle is known, 

 and not large enough to affect appreciably the magnitude of the imped- 

 ance of the resistance over the frequency range used. 



To obtain the highest degree of stability of both resistance and 

 phase angle, it has been found desirable to wind the wire on a spool 

 made of a material not afifected appreciably by atmospheric conditions, 

 for example, phenol fiber, and to immerse the complete resistance in a 

 sufficient amount of a suitable sealing compound to exclude all moisture. 

 Resistances meeting all of the requirements outlined have been con- 

 structed as described in a recent paper by one of the authors.^ Coils 

 such as described there, having a resistance of approximately 1000 

 ohms, may be constructed to have an effective inductance of less than 

 five microhenrys, and this inductance is practically independent of 

 frequency up to at least 100 kc. Coils having lower values down to 

 about 10 ohms can be made with equally small phase angles. Below 

 this value of resistance, it is more difficult to hold a low phase angle. 



Coils constructed as described may be considered to have so small 

 a change in resistance with frequency that a calibration with direct 

 current may be used without appreciable error for all frequencies at 

 which they are used. Both the variation in resistance with frequency 

 and the phase angle may be most readily measured by comparison 

 with some simple type of resistance of such geometrical form that the 

 phase angle may be readily computed. Satisfactory resistances for 

 this purpose are short lengths of fine wire of definite shape, sputtered 

 metal films on glass or other insulating material, and carbon in the 

 form of rod or film. 



Secondary Standards 



Capacitance. The value of our capacitance standards is determined 

 in terms of the prime standards of frequency and resistance. This 

 determination may be made in several ways, the following bridge 

 method being a simple and accurate one. The circuit, as shown in 

 Fig. 1, consists of two equal resistance ratio arms, a resistance and 

 capacitance in parallel in the third arm and a resistance and capaci- 

 tance in series in the fourth arm. When this bridge is balanced at 

 any particular frequency, the relations between the impedance arms 



«W. J. Shackt'ltoii, "A Sliidded A-C. Imlmtaiux- Hridgi-," .1. /. E. K. Journal, 

 February, 1927. 



