374 BELL SYSTEM TECIIXIC.IL JOIRX.-IL 



crosstalk conditions in the telephone circuits to which they are con- 

 nected. The capacity bridge and the impedance bridge previously 

 described can be used to test apparatus unbalances. 



Composite sets for superposing telegraph on telephone circuits are 

 particularh' important in respect to balance and in order to provide 

 a means for (juickly checking the balance conditions in these a special 

 form of bridge has been designed. This testing apparatus is known 

 as the composite set bridge and is of particular advantage in that it 

 provides for Cjuickly testing the balance conditions of \arious parts 



of the set as well as complete sets. Tests can be made for example 

 of I lie balance of the telegraph branches complete or of the condensers 

 and coils in these branches separately. Tests can also be made of the 

 balance of the grounded branches or of the series line condensers 

 of the set. 



To illustrate the operation of this bridge, F"ig. 19 shows the arrange- 

 ment for testing the balance of the series condensers in a composite 

 set. Two arms of the bridge A and B consist of fixed resistances 

 connected together by a slide wire resistance. The series line con- 

 densers of the composite set, Ci and d, then become the other two 

 arms of the bridge. When a source of alternaHng current is con- 

 nected as shown, a condition of minimum tone in the receiver ob- 

 tained by adjusting the position of the contactor on the slide wire 

 indicates when the bridge is balanced. The slide wire is calibrated 

 to read the jjercentage unbalance of the condensers C\ and Cj directly. 



Crosstalk and Noise Measurements. Circuit unbalance conditions, 

 such as described in some of the previous tests, are often very detri- 

 mental to telephone transmission in that they cause crosstalk between 



