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tire half-length sleeve n, then wrapped once around the insulator, and finally run back through the sleeve, which is then given one and one-half turns.
Transposing Line Wires.—Telephone instruments are so sensitive that currents induced in the line wires from neighboring circuits produce in the telephones disturbing noises commonly termed “cross talk," unless care is taken to prevent them. To overcome disturbances of this nature, which are especially troublesome in the vicinity of electric-light and power wires, both lines of a telephone circuit must be metallic and balanced with respect to adja
Method of cent lines so that the induction from Dead-ending a
Wire them is neutralized; this balancing is done by transposing the line wires of each telephone circuit on a pole at regular intervals along the line.
The method of making the transpositions is shown in Fig. 85. Transposition pins and insulators and McIntire sleeves are employed. Supposing the line wires m m and nn of a complete metallic circuit are to be transposed, these wires are cut and the two ends of each are dead-ended on the transposition insulator corresponding to that line, the upper groove of the insulator being used for one end and the lower groove for the other end. The four terminals of the two line wires are then
cross-connected by the sleeves r and s so that instead of m m and nn being continuous, m n and n m are continuous.
Four complete metallic circuits A, B, C, and D, Fig. 86, have thus been transposed at a, b, c, d, etc. Considering, for example, the circuits A and B, of which m and n are the conductors of the former and s and r those of the latter, it is obvious
Fig. 85.—Method of Transposing Line Wires
that if a current in n induces a current in s, the current in the other wire m of circuit A will induce in s a current equal in value and opposite in direction to the first induced current if the transposition a be made. The two induced currents will, therefore, neutralize each other, removing the cause of the disturbances, and in the same manner the other transpositions shown will prevent inductive disturbances in the other parts of
the circuit. In circuit C the wires should be
In addition to the desirability of thus having
Fig. 86.—Relative Locations of Transpositions in Four
resistance, the same insulation resistance, and the
Drop Wires.—The drop wires running from the
consist of a pair of No. 14 B. & S. gage hard-drawn copper wires, tinned, twisted, and insulated either with okonite or with three separate, closely woven braids of cotton impregnated with a moisturerepellent compound. The thickness of the rubber compound in the former case should be 'z-inch, and of the furnished cotton insulation in the latter case not less than 15-inch. In place of a twisted pair, however, bare open wires may be used. The drop wires are joined to the line wires as shown in Fig. 87, A being the method employed for a series connection, and B that for a bridging connection. McIntire sleeves, c, are used in the former case, and clamps s in the latter case.
Protection.—Pole lines should be protected from lightning by placing on every tenth pole a lightning rod composed of No. 6 galvanized iron wire. This should extend at least 12 inches above the
Fig. 87.—Method of Joining Drop Wires to Line Wires in
Series and Bridging Connections
pole and be soldered to a ground rod or plate at the base. The wire should run perfectly straight and be fastened to the pole by galvanized-steel staples spaced i foot apart.
Poles may be protected from the weather and at the same time improved in appearance by painting them after the line is finished, with two coats of lead and linseed oil having a dark olivegreen color.
Humming of the wires may be prevented by wrapping that portion of them at the insulators with
Fig. 88.—Device to Prevent Humming of the Line Wires
soft rubber rr, Fig. 88, and over this placing a covering of sheet lead m. The tie wire s is similarly treated, and the cushion thus formed for the line wire absorbs its vibrations and so does away with the humming.
At curves and corners the line wire at the inner end of a cross-arm may, in case it becomes loosened from its insulator, fall off and, by coming in contact with other wires, do considerable damage. To prevent the wire from falling, an iron guardarm should be screwed to the crossarm so as to catch the wire in case of its insulator breaking or its tie wire Fig. 89.-A
Lea d-covered becoming loosened.
Telephone CaTelephone Cables.—These are gen- ble erally composed of No. 19 B. & S. gage soft-drawn copper wires covered with dry paper and twisted in