allowed, provided they are securely made mechanically by splicing and soldering, and provided the insulation is made equal to that of the rest of the wire by careful wrapping of tape. The capping is then put in place, and fastened by small tacks or brads. Molding has been used in which the grooves were formed in the capping without any backing. This, however, is bad practice, and should not be adopted even where the wires are laid against a wooden wall or ceiling.

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32

Fig. 316. Three-wire Molding.

The chief disadvantages of wooden molding are the facts that it is not sufficiently impervious to moisture, is liable to be crushed or punctured mechanically, and is combustible. These difficulties are overcome as far as possible by coating the molding, both inside and out, with water-proof paint, or by impregnating it with moisture repellent. It is also recommended that only hardwood molding be used. But soft-wood molding is often laid because it follows the wall line better. In the standard forms, the backing is at least three-eighths of an inch thick under the grooves, and one-half an inch between them. The capping should

Fig. 317. Right Angle Joint in molding.

set in one-sixteenth of an inch, or more, into the backing, and should lap over the grooves not less than oneeighth inch on each side. These minimum dimensions are represented in Figs. 315 and 316, but much larger sizes of molding are used for heavier wires.

Rats gnawing through a molding may destroy the insulation of the wire, and bring the copper in contact with the wood.

Wires for use in molding must have rubber insulation, at least of an inch thick; and as the size of the wire increases from No. 14 to No. 0000 the thickness of the rubber changes to inch.

In molding where one of the wires must cross over, it is

brought out through the capping and across it, so that a certain thickness of wood is between the two conductors.

Wooden Cleats. These may be looked upon as a discontinuous molding. In fact, their cross-section is practically the same. Their use is rarely to be tolerated at present, cheapness being their only recommendation. Experience has shown that it is a great mistake to attempt extreme economy in the laying of electrical conductors. The serious difficulties which arise in the shape of damage by fire and interruption of service are far more expensive in the long run than a considerable increase in first cost.

Wooden cleats have all the disadvantages stated for wooden molding, and are open to two additional objections. One of these is the fact that the wires are left exposed for a large portion of their length, and are therefore liable to be injured or to form a short circuit or ground connection by coming in contact with each other or with some pipe, nail, or other conducting body. Wooden cleats are also likely to have small splinters projecting from them which cut through the insulation of the wires, and have been found to be a source of much trouble.

- In open

work various forms of

Porcelain Knobs or Cleats. these devices are used. (Figs. 318 to 321.)

This construction

seems to be open to the same objection as the use of wooden cleats, the wires being entirely exposed between the points of support. Nevertheless, as already stated, this construction is practically the only one allowed for high-tension circuits (over 450

volts) inside of buildings. The explanation of this apparent anomaly is found in the fact that high-tension circuits are very carefully treated when brought within buildings. For example, the primary circuit of the alternating current system is rarely allowed to run more than a few feet after it enters a building, the potential being immediately transformed down to a safe value of about 100 or 200 volts. Even in such cases the high-tension wires are only permitted in the cellar or other portion of the building not generally used; in fact, the transformer is usually placed outside of the building wherever possible. The series arc lighting circuits which are also high tension (2000 to 5000 volts) are most carefully laid when brought into buildings, the path being as short and direct as possible, and located where the wires are not likely to be touched by persons or to come in contact with anything but the insulators.

They must be rigidly supported on glass or porcelain insulators, which raise the wire at least one inch from the surface wired

Figs. 322-324. Porcelain Cleats.

over, and must be kept apart at least four inches for voltages up to 750 and at least eight inches for voltages over 750.

Rigid supporting requires under ordinary conditions, where wiring along flat surfaces, supports at least about every four and one-half feet. If the wires are unusually liable to be disturbed, the distance between supports should be shortened.

Such circuits are never introduced into buildings to anything like the same extent as low-tension wires, which run in great numbers to all parts of most modern structures. The porcelain cleat is, moreover, free from the splinters which constitute a serious objection to wooden cleats..

Glass insulators may be used instead of porcelain, but the latter is usually to be preferred because it is stronger, tougher, and less hygroscopic. The statement is often made that "any material which is non-conducting, incombustible and non-absorptive may be used, for this or other similar purpose. In point of fact, porce

lain and glass are practically the only available substances which fulfill these requirements; but if any other equivalent material can be found, its use would be permitted.

For concealed "knob and tube" work: the wires are run on the timbers and studding by means of porcelain knobs, and the wires tied to them by tie wires of equal insulation to the main wire. The wires are carried through the beams by means of porcelain tubes. These tubes are set in the beams by forcing into auger-holes, and are kept in place by the friction and by the head formed on one end of the tube. (Fig. 325.) All the porcelain devices must keep the wire one inch from the surface wired over, and the wires must be kept ten inches apart. They are preferably run on separate beams. They must be stretched so as to have no sag, and are to be supported every four feet, or even closer when necessary. This style of work is much used in country houses, where an installation for a ten-room house costs only about forty dollars.

The outlets are protected by a canvas jacket called circular loom, or by a curved porcelain tube, or even one of the beam tubes may be used for the purpose.

Mill construction: in buildings of this character mains of No. 8 wire or over, where not liable to be disturbed, may be separated 4 inches, and run from timber to timber, not breaking around, and may be supported at each timber only, otherwise, the construction in Fig. 326 or the plan of running through the timbers in Fig. 327, which cut also shows a boring-tool for this work. Unless some special tool is used, the holes will not be in line, and unsightliness as well as waste of wire is the consequence.

Wires in Plaster. Another method of concealed wiring which at one time was considered to be an ideal one, consisted in embedding the conductors in the plastering of the walls and ceilings of a building. This method could be adopted either during the original construction of the building, or in case of repairs or replastering. It was employed in many fine structures where the best construction was desired, regardless of the expense; but it was soon found that the detrimental effect of the lime in the plaster upon the insulating material rapidly injured or destroyed it.

Furthermore, the changing or repairing of a wire was rendered very difficult, necessitating the tearing away of the plastering, the trouble being aggravated by the fact that the exact location of

Interior Conduits. - The most approved method of low-tension. electrical wiring consists in providing tubes, usually laid in the floors, walls, or ceilings of a building while it is being erected, in