File:American telephone practice (1905) (14756044052).jpg

Identifier: americantelepho00mill (find matches)
Title: American telephone practice
Year: 1905 (1900s)
Authors: Miller, Kempster B. (from old catalog)
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Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

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ble. Slate of the proper quality has an advantageover marble in that it is less liable to be traversed by metallic streakswhich sometimes afford conductive paths from one part of the powerboard to another, and thus cause much trouble. There is but littledifference between the cost of the best slate and ordinary Italianmarble. Poor slate is more likely to be defective than marble. The rear view of such a switch-board is shown in Fig. 399, whichwell illustrates the method of wiring and connection furnished inlarge modern telephone plants. A full discussion of power machinery for telephone plants, thatis, of the charging and ringing machines, would involve most ofthe points which have to be considered in the design and construc-tion of dynamo electrical machinery in general. There are. how-ever, a few points in which the dynamos and motors used for tele-phone work are peculiar. The condition met with in telephone work.which is not found to an equal extent in any other field of electrical
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55S POWER PLANTS FOR COMMON BATTERY SYSTEMS. 559 engineering, is that the current delivered by the charging generatorsor consumed by any motor deriving its current from the central bat-tery, must be as nearly as possible absolutely smooth. Thefluctuations of electromotive force and current due to commutationat the brushes, or to the entrance of the various armature conductorsinto, or their passage from, the field of force of the machine, shouldbe eliminated to such an extent that no noise whatever will be heardin the talking circuits deriving currents from the terminals of thebattery during the use of the machines. Obviously, if the voltageacross the terminals of the storage battery were subject to rapid peri-odical fluctuations due to the action of the charging machines, ormotors driven from the battery, a noise would be heard in all re-ceivers connected across lines fed by the battery. It was formerly necessary, with the then available charging ma-

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