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BELL SYSTEM TECHNICAL JOURNAL 



jacks are desired to be arranged at separate primary and secondary 

 positions, and in small installations in which but a few jacks may be 

 required for all purposes. 



Fig. 18 shows the proposed assembly of a typical panel equipped 

 with voltmeter, telegraph relay and sounder, such as are usually 

 mounted above the jack field. The advantage of using the panel 



TELEGRAPH RELAY- 



Fig. 18 — Typical Assembly of Apparatus Panel for Panel Mounted Test Board 



mounting for this equipment is expected to be the same from the stand- 

 point of convenience and economy as that obtained from this method 

 of mounting the keyshelves in relation to the various jack fields. It 

 will permit the location of this equipment at any desired position and 

 eliminate duplication at positions where this apparatus is not neces- 

 sary. It will also permit flexibility in regard to the type of panel as- 

 sociated with a given jack field. 



Signaling Equipment 



The principal purposes of the signaling equipment in telephone 

 lines are (1) to permit a subscriber to signal the central office opera- 

 tor, as he does automatically in removing the receiver from the switch- 

 hook, (2) to permit the central office operator to ring the subscriber's 

 bell and (3) to permit the operators at different central offices to signal 

 each other. At repeater stations this equipment serves to pass the 

 signals around the telephone repeaters which might otherwise inter- 

 fere with their transmission. 



The switchboards in both the local and toll central offices have 

 been provided with a source of 20-cycle current for signaling, as 

 current of this frequency is suited to operate the subscriber's bell 

 directly. This frequency has also been satisfactory for operating 

 the signaling apparatus in the various local trunk circuits and in 

 short toll lines without superposed telegraph. The introduction of 

 superposed telegraph and telephone repeaters, however, has prevented 



