88 BELL SYSTEM TECHNICAL JOURNAL 



The operation of the testing equipment may be varied by suitable 

 keys, so that all the features of each sender may be tested once, or 

 so that any one feature of the sender may be tested as many times 

 as desired. 



All the equipment in the office occurs in groups, and arrangements 

 are made for readily taking out of service for readjustment any piece 

 of apparatus which may have been found to have potential trouble — 

 the other members of the group continuing to handle the calls. 



Application 



In the preceding pages there has been briefly described a switching 

 system which meets the exacting and complex requirements of tele- 

 phone service in the largest cities and in which, so far as is practicable, 

 the various switching operations are performed automatically. Only 

 such operators are required in connection with this system as are 

 necessary for handling special classes of service and certain operations 

 in connection with the interchange of calls between manual and 

 machine switching central offices during the transition period. 



Variations in the arrangements which have been described have 

 been developed and are available for use whenever the conditions 

 warrant. An illustration of this is the so-called key indicator, which 

 permits the handling of calls from manual to machine switching 

 offices without the aid of the cordless "B" operators. This is effected 

 by providing the operators in the manual offices with special keys 

 and equipment for controlling directly the selection of the subscrib- 

 er's line in the machine switching office. 



This machine switching system marks a very important advance 

 in a development which began shortly after the telephone was in- 

 vented, and which has been most vigorously prosecuted by the en- 

 gineers of the Bell System from then to the present time. Through- 

 out this entire development period the tendency has been to introduce 

 automatic methods and apparatus whenever they gave a better result 

 to the public, or whenever they were attended by an economy of any 

 kind. 



How this system works has been briefly explained. What arrange- 

 ments are provided for handling regular machine switching calls, 

 calls to and from existing manual offices, private branch exchanges, 

 etc., has been described. How the introduction of this system into 

 a telephone network is affected will now be discussed briefly. 



Obviously, the problem of introducing machine switching equip- 

 ment into such an extensive and complex structure as is the telephone 



