ELECTRICAL MEASUREMENT 



79 



standard may be made with a range of 10 henrys to a minimum of 

 two millihenrys, using an inductometer having a minimum scale 

 division of 0.1 millihenry, or the range may be any simple multiple of 

 this. Values as low as one microhenry at frequencies as high as 150 

 kc. are measured in this way. 



By connecting the resistance in one arm of the bridge and a capaci- 

 tance in series with an inductance in the other arm, we may use it to 

 indicate resonance, and if we measure the frequency we may use this 

 method for the comparison of capacitance with inductance. This is 

 the method actually used for the calibration of the inductance standard 

 used with the bridge. The bridge may be used for the comparison of 

 capacitance. The bridge described later for the measurement of 

 capacitance, however, has certain special features which make it 

 peculiarly adapted to the measurement of capacitance and conduct- 

 ance. 



Inductance with Superposed Direct Current. In telephone work, 

 it is often of value to know the performance of apparatus, particularly 

 of iron core impedances, when used at telephone frequencies while'at 

 the same time carrying direct current. The bridge shown in schematic 

 form in Fig. 6 will measure the inductance of the coil at audio frequency 



DC. SOURCE 



D.C. AMMETER ^ D 



Fig. 6 — Bridge circuit for measuring impedances with superposed direct current 



THERMOCOUPLE 

 MILLIAMMETER 



with a direct current flowing through it. As shown in the figure, 

 the direct current is kept out of all of the arms of the bridge except one 

 ratio arm and the test arm, by means of condensers, and the alternating 

 measuring current is separated from the direct current by means of 

 a choke coil. None of these added features affect the bridge balance 

 except the capacitance in the standard arm, and this is made large 



