. Railroad construction. Theory and practice. A textbook for the use of students in colleges and technical schools . ngth should therefore be inches in-stead of 22 inches. If the mean temperature was very different(say in Florida) some higher temperature should be taken asnormal, 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 ar
. Railroad construction. Theory and practice. A textbook for the use of students in colleges and technical schools . ngth should therefore be inches in-stead of 22 inches. If the mean temperature was very different(say in Florida) some higher temperature should be taken asnormal, 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. 310. Track circuit for automatic signaling. The severalsystems of automatic signaling differ in the minor details, butnearly all of them agree in the following particulars. 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 other. Fig. 172.—Deflecting-rods. line of rails. To avoid the excessive resistance which wouldoccur at rail joints which may become badly rusted, a wire §310. BLOCK SIGNALING. 361 suitably attached to the rails is run around each joint. Inorder to insulate the rails of one sec- _^ tion from the rails at either end andyet maintain the rails structurally con-tinuous, the ends of the rails at thesedividing points are separated by aninsulator and the joint pieces are eithermade of wood or have som3 insulatingmaterial placed between the rails andthe ordinary metal joint. The boltsmust also be insulated. When therelay is energized by a current, itcloses a local circuit at the signal-station, which will set the signal thereat safety. Tha resistance of therelay is such that it requires nearly thewhole current to work it and to keepthe local circuit closed. Therefore,when there is a
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