Identifier: streetrailwayrev05amer (find matches)
Title: The street railway review
Year: 1891 (1890s)
Authors: American Street Railway Association Street Railway Accountants' Association of America American Railway, Mechanical, and Electrical Association
Subjects: Street-railroads
Publisher: Chicago : Street Railway Review Pub. Co
Contributing Library: Carnegie Library of Pittsburgh
Digitizing Sponsor: Lyrasis Members and Sloan Foundation

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e 700 feet away where connection is made with the cables to be tested. A tele-phone also connects these points. For the sake of those not familiar with cable testingwork of this kind, a brief outline of the method pursuedis here given. Insulation resistance like any other resist-ance is inversely proportional to the current that will flowthrough it with any given electro-motive force. Theproblem then is to have some constant source of currentwhich will give an unvarying voltage or electro-motiveforce and to compare the current that will flow throughthe insulation to be measured, with a known resistance.To give a constant electro-motive force, the 100 cellsof chloride battery are used. The current required intesting is so exceedingly small that the battery voltage re-mains constant over long periods and deteriorates verylittle. The deflections of the galvanometer, except onvery large deflections, are proportional to the current flow-ing through it. Before making a cable test the galva-
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TESTING LABORATORY WEST CHICAGO STREET RAILROAD. nometer is connected to the battery in series with a knownresistance and the deflection noted. The resistance incircuit multiplied by the deflection will give the megohmsresistance that are in circuit for one degree deflection ofthe galvanometer with a constant batter) force. Withthe galvanometer now used, and 100 cells in series, onedegree deflection will be obtained with about 200,000megohms in circuit. This number is called the constant.As the current that will flow and consequently the galva-nometer deflection in making any test is inversely propor-tional to the resistance, the constant must be divided bythe deflections obtained from the galvanometer durinthe test of a cable in order to get the insulation resistance.The constant is determined before table testing is begunevery day and is redetermined at intervals during thetests. The insulation resistance of the leads of course hasto be determined separately every hour or two and th

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