File:The practical telephone handbook and guide to the telephonic exchange (1906) (14755884202).jpg

Identifier: practicaltelepho00pool (find matches)
Title: The practical telephone handbook and guide to the telephonic exchange
Year: 1906 (1900s)
Authors: Poole, Joseph
Subjects: Telephone
Publisher: New York, Macmillan Co.
Contributing Library: Northeastern University, Snell Library
Digitizing Sponsor: Boston Library Consortium Member Libraries

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otor is in motion the current is not according to this law(or rather appears not to be so), as the faster the armaturerevolves the less is the current in the armature coils. This isowing to the fact that the revolutions of the coils in themagnetic field create a back E.M.F., the voltage of which isin proportion to the speed, and which opposes the originalor impressed E.M.F. As the full current at starting, ifput on suddenly, may be likely to damage the coils by heat,and also put a great mechanical strain upon them, a startingswitch is used. This, by inserting a resistance in serieswith the machine, and then gradually cutting it out as themotor gets up speed, gives a gradual increase of E.M.F., andtherefore current, to the coils. Generator-Field Rheostat.—The generators used are usually shunt wound machines, and a rheostat is included in thefield-magnet circuit, so that by inserting more or less outsideresistance the current through the- magnet coils can be varied THE POWER PLANT 315
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3i6 PRACTICAL TELEPHONE HANDBOOK as desired, and the E.M.F. generated in the machine can beraised or lowered at will. Double-Pole Switches.—These are heavy switches, used toconnect the generators to the battery when the machine hasattained the proper speed for charging—viz. so as to givean E.M.F. of about 30 volts. If connected before the voltagewas equal to that of the battery a reverse current would besent through the machine and the polarity of the field-magnetsreversed. To guard against such an event, and also againstan excess of current due to some accidental short-circuit,a Circuit Breaker is inserted in the main circuit. This isa spring-lever switch held by a catch, which is released eitherby a very low-resistance electro-magnet in the main circuitoperating when the current reaches a certain excess point,or by a comparatively high - resistance electro - magnetenergised by means of current connected by a polarisedrelay in the main battery circuit, which only operates whenthe

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