86 BELL SYSTEM TECHNICAL JOURNAL 



balances in which we are interested. It consists of an input and an 

 output transformer, two equal resistance ratio arms, a variable air 

 condenser of the three-plate type, four binding posts for connecting 

 the four conductors of the quad, and switches for making the various 

 connections. By means of the switches, the cable conductors are 

 connected to the circuit in such a way that the reading of the air 

 condenser when a balance is obtained indicates directly the unbalance, 

 either side-to-side or phantom-to-side, according to the switch posi- 

 tions. This circuit when used as a laboratory instrument is capable 

 of measuring capacitance unbalance as low as 1 mm f- 



Attenuation and Gain. So far, we have discussed the measurement 

 of the fundamental impedance characteristics of apparatus. When 

 the component parts have been found to meet their individual im- 

 pedance requirements and are assembled to form the completed 

 apparatus, it is desirable to have tests made of the over-all performance 

 of this apparatus. In a large number of cases, the requirement of 

 greatest importance is the attenuation frequency characteristic. It 

 is fairly obvious that this characteristic, of all apparatus used in 

 telephone lines, is of interest, and this is particularly true of all types 

 of filter circuits which are designed primarily for the purpose of 

 furnishing definite attenuation frequency characteristics. These meas- 

 urements are particularly required on apparatus used in carrier-current 

 telephony and telegraphy. 



From the very nature of the measurements, it is difficult to obtain 

 a null method of measuring attenuation. The most direct method is 

 to measure the input and the output of the apparatus under test simul- 

 taneously, from which the attenuation may be computed. The prac- 

 tical difficulty in doing this is to measure the extremely small outputs 

 which are obtained from apparatus having high attenuations, where 

 the characteristic must be obtained with the normal input, which is 

 usually low. In general, it has been found necessary to use some form 

 of amplifying device in the output circuit and it has not been found 

 desirable to rely on the constancy of amplification of this device. 

 Accordingly, the usual method used for the measurement of attenua- 

 tion is a substitution one. The circuit is shown in Fig. 11 A. There 

 are two branches in this circuit, one of which includes the apparatus 

 under test and the other, a variable standard attenuator. The 

 output of each branch is arranged to connect either to a detector 

 of impedance Zi equal to the impedance of the standard attenuator 

 or to a fixed impedance of the same value. If the apparatus under 

 test has the same impedance as the standard attenuator, the input 

 impedances Zi and Z3 are made equal and the matching impedance Z-j 



