lU.r.CTKICAl. TESTS .IXD TIII-.IK .WrUCITlONS 371 



condition i>f balanre indicated !)>■ ininiimmi time in the receiver, tlie 

 effective resistance of the circuit is ^;i\cn ditcciK 1)\ the value of lh<; 

 variable resistance R, and the inductance by the value of L. For any 

 particular frequencN'/ at which a measurement is made, the reactance 

 of the circuit can be computed from tiic \.i!ue of /, and lAprc^xd in 

 ohms by the formula 



Reactance = 2jr fL. 



The impedance of the circuit expressed in ohms ise(|ual in the vk icui.il 

 sum of the eflfective resistance R and the reactance. This relation 

 is made use of in practice when it is desired to express the impedance 

 of circuits in roimd numbers without reference to its component 

 parts. Generally, howe\'er, in the practical applications of impedance 

 measuring in maintenance work, the resistance and inductance com- 

 ponents can be used directly to the best advantage without combining 

 them or expressing the inductance readings in terms of reactance. 



One of the most important applications of impedance measurements 

 is the determination of the characteristic impedance of telephone 

 circuits at the various frequencies invohed in the transmission of 

 telephone currents. Measurements of this kind, when applied to 

 equipment circuits such as telephone repeaters, balancing networks, 

 etc., and to the line circuits themselves, tell a great deal in regard to 

 the efficiency of these circuits for the transmission of speech. They 

 are ver\' important, therefore, in checking up the installation of 

 certain circuits in the plant and making sure that the jiropcr im- 

 pedance relations are obtained. 



Fig. 17 shows the results of impedance measurements on a loadci! 

 19 gauge cable circuit within a range of frequencies from 'M)U c\cks 

 to 2.3(K) c\cles. The effective resistance values and the values of the 

 reactance comp<inents are indicated by the cur%'es. The inductance 

 values are negative which means that the circuit tested had capacitive 

 reactance throughout the range of frequencies used. \\'hen the meas- 

 urements were made the distant terminal of the circuit was terminated 

 by an impedance approximating the characteristic impedance of the 

 circuit in order to give the effect of an infinite length of line. If the 

 above circuit is used for 2-way telephone repeater operation it is ncces- 

 sar\- that the repeater balancing networks ha\e impedance character- 

 istics similar to the lines which they balance in order that the maxinnmi 

 repeater gain with gcxxl quality be obtained. 



Measurements such as described above, in addition to giving a 

 picture of the effective resistance and reactance of circuits at different 

 frequencies, also provide a means for locating the irregularities and 



