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No. 16 at least 3 inch thick. Copper wires in sizes of 19 and 20 are also used for this kind of wiring, but they are not considered so desirable. A table of dimensions, resistances, and current-carrying capacities of copper wires including No. 16, 18, 19, and 20 B. & S. gage is given in the Appendix. It will be noticed from this table that the currentcarrying capacity of the No. 18 is 5 amperes. As far as the current-carrying capacity is concerned,
much smaller wire could be used, as telephone currents for the work considered in this book seldom exceed to ampere, but owing to the mechanical weakness of smaller wire ren
dering it liable to be broken BRUNT
or stretched, no smaller sizes than No. 18 should ordinarily
be employed. In making conFIG. 36.- Porcelain Insulator for Interior nections, the insulation of the Telephone Wiring
wire must be thoroughly scraped off and the wire made clean and bright at the points of contact. Whenever two wires are joined together, the connection should be soldered.
Wires must never be installed nearer than i inch to any kind of piping, and if it be necessary to cross pipes upon which moisture is liable to form, the wires should be led over rather than under them. Porcelain insulators, No. 5, such as are shown in Fig. 36, should preferably be used to support the wires, although the latter may be fastened in place in any neat, secure, convenient, and workmanlike manner. Short single lengths of the insulated telephone wire can be used as tie wires in case porcelain insulators are employed. A porcelain insulating tube, Fig. 37, placed with the head c at the entrance hole, affords excellent protection for the wires through walls and other partitions; continuous flexible tubing or conduit is next best; but if neither tube nor tubing is avail
Fig. 37.-Porcelain Tube for Protecting Wires from
Partitions and Walls
able, the wires should be wrapped with two layers of insulating tape.
Fishing for the wires between walls or under floors is done by boring 2 holes in the wall or floor along the desired path, pushing a flat spring wire from one hole toward the other, and catching it through the latter hole with a wire hook. By means of the spring wire, the flexible conduit containing the telephone wires may then be drawn into position. The flexible conduit or extra cov
ering of the wire should extend in a continuous length between the entrance and exit holes, projecting beyond them about 1 inch in each case. Telephone wires must never be installed nearer than 6 inches to any electric light or power wire in a building, unless incased in porcelain tubing or flexible conduit so secured as to prevent its slipping out of place.
The wires leading out of the building should be No. 14 weather-proof wire, protected from the
Fig. 38.-Porcelain Drip Tube for Protecting the Wires from the House Wall and Preventing Water from Following the Wires Inside
house walls either by a straight porcelain tube like that in Fig. 37, placed so as to slope slightly upward from the outside, or by a porcelain drip tube as shown in Fig. 38, placed with the head c outside the building. The slanting of the tube in the former case, or the drip loop in the latter case, effectually prevents water from outside following the wires within.
Figs. 39 and 40 show, respectively, one method of wiring between the house and the first pole in
the case of a single or grounded line and in the case of a double or complete metallic line. In both illustrations, a represents the porcelain tube, c
the conductors leading to the line wires, n the line wires, s the ground wire, t the ground rod, e porcelain insulators, and r the soldered connections.
The Wiring of the Protective Apparatus does not always follow that shown at a in Fig. 32; in fact, in recent practice the lightning arrester is seldom mounted on the telephone set but is placed on a non-combustible, non-absorptive insulating base immediately inside the building at the point where the line wires enter. As lightning protection is necessary only from those discharges coming in from the line wires, this latter method is preferable to the former in that it conducts the discharges to ground by a more direct path.
Although in many telephone installations a lightning arrester which will operate with a difference of potential of 500 volts is the only protection provided at a station, the best practice consists in using in addition to the lightning arrester a fuse and a heat coil on each side of the line. The fuse is intended to open the telephone circuit in case the line wires become crossed with electriclight or power circuits, the stronger current of these circuits melting the fuse wire in passing through it. The heat coil is also intended to warm up and melt out with a current of sufficient strength to endanger the instruments if continued for a long time, but of so low a voltage that it would not traverse the lightning arrester, and of so small an amperage that it would not melt the fuse wire