80 BELL SYSTEM TECHNICAL JOURNAL 



enough (26 fx f.) to have an impedance small compared with the 

 impedance measured. In any case, a correction may be made by 

 taking first a zero reading which will be slightly positive due to the 

 inductance necessary to compensate for the capacitance in this circuit. 

 This correction will vary with frequency but at 1800 cycles, for 

 instance, with 26-^ f. capacitance, the correction is only about 0.3 

 millihenry and the inductances measured are usually considerably 

 larger than this. 



The circuit is extremely simple and convenient to use. The values of 

 alternating current and direct current can each be measured separately 

 outside of the bridge circuit and the inductance standards do not need 

 to be constructed to carry the direct current. The only part of the 

 bridge required to carry the direct current is one ratio arm and, in 

 consequence, it is a comparatively simple matter to construct such a 

 bridge to carry several amperes of direct current. Where very high 

 direct currents are required, the ratio arms may be reactances wound on 

 a single core, instead of resistances, thus reducing the loss due to the 

 passage of the direct current. 



Flutter. In telephone circuits used for joint telephone and 

 telegraph service, it is desirable to know the effect of the telegraph 

 impulse on the telephone frequency inductance and effective resistance 

 of the loading coils used on the lines. This effect, known as "flutter," 

 with a method of measuring it, is described in detail by Fondiller and 

 Martin.^ The measuring circuit consists of a double bridge, the inner 

 one consisting of two similar loading coils on which the flutter effect 

 is to be measured and two other coils of comparatively high impedance 

 approximately equal in value and which have negligible flutter effects, 

 the four coils being connected to form a balanced bridge. The low 

 frequency corresponding to the telegraph impulse is introduced at 

 two diagonal corners and the other two corners, which are at a common 

 potential with respect to the low frequency, are connected to the usual 

 test terminals of an impedance bridge of the type already described. 

 With no low-frequency current passing through the coils, a continuous 

 balance may be obtained on the main or high-frequency bridge using 

 an audio frequency input. From this, the normal effective resistance 

 and inductance of the coils may be obtained. 



' When the low-frequency current passes through the coils, the 

 inductance and effective resistance are different for every point of the 

 low-frequency cycle. Thus, only an instantaneous balance of the 

 outer bridge is possible. This instantaneous balance for any particular 

 point in the low-frequency cycle may be made by the use of an electro- 



'W. KondillcT aiul W. 11. Martin, 'Jntiisctctions oj the A. I. K. K., 1<)21, \'oi. 40, 

 p. 553. 



