Telephone principles and practice

 
9781130281361: Telephone principles and practice
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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1904 Excerpt: ... generator at No. 1 would have to furnish current sufficient to ring all bells, or four times the current necessary to ring one bell; also the ringers at station Nos. 1, 2 and 3 would have to be wound with resistance high enough to partly choke up the paths leading to the earth and in this way cause some of the current to be forced on to No. 5. A party line often contains several miles of wire and hence may offer considerable resistance. In such cases the generator must also produce a pressure great enough to force the current to station No. 4. In other words, a bridging generator must furnish a large output of both current and pressure, which calls for larger generators than the series line. In order to talk well over a bridging line, the intervening stations must have high wound ringer coils for, as we have seen, the more turns there are in a coil the greater its impedance and the harder it is for alternating currents of high frequency to pass through it; hence by putting on plenty of wire, the coils will shut out the talking currents, and allow them to pass on over the line. Coils wound to about 1,600 ohms resistance are found to shut out the telephonic currents almost completely. The ground currents, spoken of in the series grounded system, are also annoying in this one, though not to the same extent; consequently, most grounded systems are being rapidly changed over to full metallic circuits, and it will be a matter of only a few years when the majority of party lines will be of this kind. 196. Bridging Metallic.--This system is represented in Fig. 187, in which the instruments are bridged across the two line wires, the middle binding post, on each instrument being grounded as a protection against lightning. This system has come into general use during...

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