. American telephone practice . 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 t
. American telephone practice . 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-. FIG. 400.—IMPEDANCE COIL. chines, to place a heavy impedance coil in the charging leads to re-duce the fluctuations in current and consequent noise in the talking-circuits, and even with this it was often necessary to charge only insuch hours as the exchange was least busy, that is, at night. Bymeans of the machines now available, however, this is not neces-sary; the machine may run in the daytime or whenever the objectionable impedance coil, which, of course, consumes acertain amount of energy, is done away with. An impedance coil foruse with machines not suitable for giving sufficiently smooth currentis shown in Fig. the principal factor in the design of charging machines 560 AMERICAX TELEPHONE PRACTICE. accomplishing this result is the use of a much greater number ofcommutator bars than would be required for almost any other typeof direct current machine. The brushes on such machines with theyery narrow commutator bars used cover approximately three com
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