. American telephone practice . we have a local batteryexchange, where each sub-station apparatus contains a local bat-tery for supplying current to his transmitter, or we may have a com-mon battery or central energy exchange where all sources of energyfor both calling and talking are located at the central office. In thelocal battery exchange the sub-station apparatus usually comprises TELEPHONE EXCHANGE IN GENERAL. 173 a magneto generator for enabling the subscriber to call the centraloffice, for which reason such an exchange is often spoken of as amagneto exchange. It may be said that the g
. American telephone practice . we have a local batteryexchange, where each sub-station apparatus contains a local bat-tery for supplying current to his transmitter, or we may have a com-mon battery or central energy exchange where all sources of energyfor both calling and talking are located at the central office. In thelocal battery exchange the sub-station apparatus usually comprises TELEPHONE EXCHANGE IN GENERAL. 173 a magneto generator for enabling the subscriber to call the centraloffice, for which reason such an exchange is often spoken of as amagneto exchange. It may be said that the grounded circuit or common return ex-changes are gradually giving way to the exchange using only com-plete metallic circuit lines, and that the local battery or magnetoexchanges are gradually giving way to the common battery or cen-tral energy exchange wherein all sources of electrical energy arelocated at the central office. Particularly is this true in large ex-changes. The magneto or local battery exchange still has, however,. FIG. 153.—THREE-OFFICE EXCHANGE. a wide field of usefulness in small exchanges, and will probably neverbe completely supplanted. In an automatic exchange wherein no operators are supposed tobe employed, each subscriber must have at his own station, mechan-ism which, when properly operated by him, will continue the circuitof his line through auxiliary circuits and mechanisms at the centraloffice, to the line of the subscriber desired, and after this connectionis made to ring the bell of that subscriber. Further means must beprovided by which the subscriber may clear his line from such con-nections after he is through talking. 174 AMERICAN TELEPHONE PRACTICE. It may be said that up to the present time the automatic exchangehas come into only very limited use, although there is at present amovement on foot which promises to have installed automatic ex-changes in several large cities of this country within a comparativelyshort time. The success of this mo
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