. American telephone practice . disc are of such resistancethat the entire fifteen of them equal the resistance of one coil on the 568 AMERICAN TELEPHONE PRACTICE. front disc. The field resistance may then be adjusted roughly on thefront disc, and then by manipulating the lever of the rear disc incre-ments may be added until the desired resistance is obtained. In the double-faced rheostat shown in Fig. 411, a total of 225 stepsis obtained on a rheostat of the diameter of an ordinary fifteen-pointrheostat. The same general rules that apply to the care of dynamo electricalmachinery in general wi
. American telephone practice . disc are of such resistancethat the entire fifteen of them equal the resistance of one coil on the 568 AMERICAN TELEPHONE PRACTICE. front disc. The field resistance may then be adjusted roughly on thefront disc, and then by manipulating the lever of the rear disc incre-ments may be added until the desired resistance is obtained. In the double-faced rheostat shown in Fig. 411, a total of 225 stepsis obtained on a rheostat of the diameter of an ordinary fifteen-pointrheostat. The same general rules that apply to the care of dynamo electricalmachinery in general will apply to charging and ringing machinesin telephone work, but on account of the nicety with which thesemachines are made and adjusted, if of the proper type, an evengreater amount of attention should be paid to their care. In tele-phone work special stress should be laid on the necessity of main-taining the commutator brushes in good order, as carelessness inthis matter is one of the most prolific causes of disturbing noises. FIG. 411.—HOLTZER-CABOT RHEOSTAT. caused by machines in the telephone lines. The brushes whenproperly set on a properly designed machine, will not cause noise inthe lines even when no choke coils are placed in the charging has been stated that the brush should be wide enough to bear onabout three complete segments on the commutator, and they shouldbe fitted to the commutator in such manner that the entire sur-face of the brush makes contact with the commutator surface. A good way to fit a brush after the brush-holder is properlyplaced is to lay fine sand-paper on the commutator, rough side out,and then draw it under and brush in the direction of the rotation ofthe armature, the brush being pressed against the sand-paper by thespring of the brush-holder. During the first rough adjustment of POWER PLANTS FOR COMMON BATTERY SYSTEMS. 569 the brush in this manner, no harm will be done if the sand-paperis pulled back and forth under the brush, and t
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