. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . normal, so that the extreme range above and below the normalshall be approximately the same. Guides around curves and angles. When wires are requiredto pass around curves of large angle, pulleys are used, and alength of chain is substituted for the wire. For pipes, whenthe curve is easy the pipes are slightly bent and are guidedthrough pulleys. When the angle is sharper, angles areused. The operation of these details is self-evident from aninspection of Fig. 172. 422 RAILROAD C
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . normal, so that the extreme range above and below the normalshall be approximately the same. Guides around curves and angles. When wires are requiredto pass around curves of large angle, pulleys are used, and alength of chain is substituted for the wire. For pipes, whenthe curve is easy the pipes are slightly bent and are guidedthrough pulleys. When the angle is sharper, angles areused. The operation of these details is self-evident from aninspection of Fig. 172. 422 RAILROAD CONSTRUCTION, §394. 394. Track circuit for automatic signaling. The fundamentalprinciple of the track circuit method of indicating a track obstruc-tion or breakage, using direct current, is as follows: A currentof low potential is run from a battery at one end of a sectionthrough one line of rails to the other end of the section, thenthrough a relay, and then back to the battery through the otherline of rails. To avoid the excessive resistance which wouldoccur at rail joints which may become badly rusted, a wire. Fig. 172.—Deflecting-rods and Angle. suitably attached to the rails is run aroimd each joint. Inorder to insulate the rails of one section from the rails at eitherend and yet maintain the rails structurally continuous, the endsof the rails at these dividing points are separated by an insulatorand the joint pieces are either made of wood or have someinsulating material placed between the rails and the ordinarymetal joint. The bolts must also be insulated. Whien therelay is energized by a current, it closes a local circmt at thesignal-station, which will set the signal there at safety.^ Theresistance of the relay is such that it requires nearly the wholecurrent to work it. and to keep the local circuit closed. There-fore, when there is any considerable loss of current from onerail to the other, the relay will not be sufficiently energized, thelocal circuit will be brok
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