File:American telephone practice (1905) (14569793540).jpg

Identifier: americantelepho00mill (find matches)
Title: American telephone practice
Year: 1905 (1900s)
Authors: Miller, Kempster B. (from old catalog)
Subjects:
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Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

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removed.There are several methods of doing this, one of which is to pumpthe gas out with an inverted umbrella made specially for the pur-pose. The umbrella is lowered into the manhole while closed andthen suddenly withdrawn, this opening the umbrella and lifting outthe gas. Another way of clearing manholes from gas is to placea cloth screen above the manhole and on the side opposite to that UNDERGROUND CABLE CONSTRUCTION. 847 from which the wind is blowing. The wind on striking the screenis deflected downward, thus causing an eddy which removes thegas from the manhole. Very serious explosions have been causedby the collection of gas in manholes, which becomes ignited eitherby an electric spark or by the torch of a workman. One of the most serious difficulties in connection with under-ground cable work is that brought about by electrolysis, due tothe action of stray earth currents, usually due to the ground returnof electric railways. It is found that the electrolysis occurs at points
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FIG. 622.—REEL TRUCK. where a current flowing along the cable sheath leaves the sheathand enters the ground. At this point oxygen is liberated, which,with the chemicals in the earth, rapidly corrodes the lead sheath. Ofcourse, the construction of conduits, composed of insulating mate-rial, will do something towards the alleviation of this trouble. Frequent tests should be made, however, on all cable systems todetermine the polarity of the cable sheaths with respect to sur-rounding conductors. The tests for this purpose may be made asfollows: Two brass rods about six feet long should be provided, each 848 AMERICAN TELEPHONE PRACTICE. having a steel contact at one end. Between these two rods shouldbe connected by flexible wires a portable voltmeter—one reading tofive volts will usually be found most suitable. The test should bemade at the manholes, these being the most available points forreaching the cable. One of the steel contact points should then beplaced in firm contact with t

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