. The practical telephone handbook and guide to the telephonic exchange . he arrangements adopted by the National Telephone the joining up of a single-extension instrument, givingintercommunication between all points, and so arranged thatwhen the main is speaking to the extension, conver-sation cannot be overheard at the central office. To accom-plish this a triple form of intermediate switch is used, the 136 PRACTICAL TELEPHONE HANDBOOK principle of which will be readily understood from the dia-gram. The insulating plunger is so pivoted that it may bepressed between any one of the thre
. The practical telephone handbook and guide to the telephonic exchange . he arrangements adopted by the National Telephone the joining up of a single-extension instrument, givingintercommunication between all points, and so arranged thatwhen the main is speaking to the extension, conver-sation cannot be overheard at the central office. To accom-plish this a triple form of intermediate switch is used, the 136 PRACTICAL TELEPHONE HANDBOOK principle of which will be readily understood from the dia-gram. The insulating plunger is so pivoted that it may bepressed between any one of the three pairs of switch-springs—1, 2 and 3. This causes the springs to be lifted from the innerto the outer spring contacts. Normal; Exchange to Main and Extension—In thenormal position, as shown in the figure, the extension andmain instruments are connected in bridge to the exchangeline, so that the exchange is signalled whenever either of thereceivers is lifted from its support, the other station not beingdisturbed. No, 1 Main to Exchange.—When the plunger is pressed. To MAIN INST. Fig. 150.—Connections for Single Extension with Intercommunica-tion-Magneto Calling in No. 1 switch the extension line is cut off the exchange andjoined to the extra bell through the condenser, while the main remains connected to the exchange line withoutthe possibility of being overheard by the extension, as wouldbe the case in the normal position. No. 2 Main to Extension.—When No. 2 switchis used the main instrument is connected to the extensionline, and the exchange line is joined to the condenser andextra bell. Signalling between the two stations is done bythe special magneto-generators. These generators are fittedwith an extra cut-out contact, so that when the handle isturned the curved spring leaves the outer contact and makeson the inner one. Two dry cells are inserted, as shown, so SWITCHES AND EXTENSION INSTRUMENTS 137 that the speaking may be independent of the exchange currentsupply.
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Keywords: ., bookcentury1900, bookdecade1900, booksubjecttelephone, bookyear19
