I.I.ECIKIC.U. TLSTS .l\l> IlllUi .iri'UC.lllOXS 



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ririiiii>. Also foreign nirri'iUs, iiidiuiMl from sii[)|)ly liiu-s, priMluce 

 noiso which has much the same effect as inserting a transmission loss. 

 The ma^niluile of noise produced in this \va\' is de|H'ndent amon^ 

 other thinijs, on the balance conditions of hoth the suppK' and tele- 

 phone circuits. 



The determin.ition of tlu' m.ignitude of crosstalk and of noise 

 currents can be made by relatively simple measurements. In practice 

 crosstalk tests, which also gi\e an indication of the balance conditions 

 of circuits, can be made more quickly than impedance unbalance tests, 

 although they do not give a location tlirectly of an\' troubles which 

 may e.xist. The usual procedure then is to make noise and crosstalk 

 tests on circuits, and in those cases where the measurements indicate 

 that impro\ement is desirable some of the direct current or alternating 

 current methods previously described are applied to locate the cause. 

 The simplified circuit arrangement of the test set commonly u.sed for 

 measuring crosstalk between two circuits is shown in Fig. 20. 



.An alternating current .source generalK' of conijilex wa\e shape to 

 simulate voice currents is connected to a switch in the set so arranged 

 that its voltage can be impres.sed either on a telephone circuit known 

 as the "Disturbing Circuit" or on a measuring shunt known as a 

 "Crosstalk Meter." The other side of the shunt is connected through 

 a Wheatstone bridge arrangement to a second telephone circuit known 

 as a "Disturbed Circuit." Shielded transformers are used in the set 

 as shown for connection to the circuits under test, these transformers 

 l)eing designed to give the proper impedance relations re(|uired by 

 the different types of circuits .net with in practice. The Wheatstone 

 bridge arrangement is primarily for the purjKise of allowing any noise 

 currents which may be present in the disturbed circuit to be impressed 

 on the observing receivers either when these are used to listen to the 



