ISO BELL SYSTEM TECHNICAL JOURNAL 



In general, the unknown impedance will have capacitances to ground 

 and the effect of these will be properly included in the measurement 

 only when certain conditions as determined by the nature of the 

 apparatus are fulfilled. From this standpoint the impedances usually 

 encountered are of three general classes: (1) Those having ground 

 admittances negligibly small in comparison with the direct terminal- 

 to-terminal admittance; (2) those having appreciably large admit- 

 tances to ground approximately balanced with respect to the two 

 test terminals; (3) those having one terminal directly grounded, the 

 other having an appreciably large ground admittance. 



In measuring apparatus of the first type it is evident that since in 

 connecting it to the bridge circuit no additional ground admittances 

 are introduced, the balance between those previously existing can be 

 made without reference to the test impedance. The connection of an 

 impedance of either of the other types will, however, introduce addi- 

 tional ground admittances into the bridge system, which, unless pre- 

 cautions have been taken, may cause the result to be something other 

 than that which is wanted. In general, the desired test is that which 

 gives the effective impedance applying to the apparatus as it is used. 

 In the case of impedances having balanced admittances to ground, 

 this is the effective value of the direct, terminal-to-terminal impedance 

 as modified by the effect of the two ground admittances acting simply 

 in series with each other. This condition is obtained when equal 

 currents flow in each of these admittances, or, what is equivalent, 

 when the electrical potentials of the terminals are balanced with 

 respect to ground potential. To obtain this condition, when the 

 impedance is being tested, the bridge terminals to which it is connected 

 must likewise be balanced with respect to ground potential; that is, 

 ground potential must be at the midpoint of the unknown impedance 

 arm. If the only admittances to ground of the bridge system are 

 those of the junction points (as is the case in Fig. 8), the potentials 

 of these points with respect to ground are entirely determined by these 

 admittances. To make any two points, such as the terminals of the 

 unknown arm, have equal potentials to ground, it is sufficient to con- 

 centrate all of the ground admittances to these or other equipotential 

 points and then balance the admittances from each. Referring to 

 Fig. 9, if the testing current is applied at the points A and C, this 

 condition is realized as shown by concentrating all ground capacitances 

 at junction points B, C and D, and making the sum of the capacitances 

 of junction points B and D equal to that of junction point C. This 

 follows from the fact that when the bridge is balanced the junctions 

 B and D are equipotential points. The mid-point of arm CD is now 



