h. Wires run as above immediately under roofs, in proximity to water-tanks or pipes, will be considered as exposed to moisture.

i. When from the nature of the case it is impossible to place concealed wire on non-combustible insulating supports of glass or porcelain, the wires may be fished on the loop system, if encased throughout in approved continuous flexible tubing or conduit.

j. Wires must not be fished for any great distance, and only in places where the Inspector can satisfy himself that the above rules have been complied with.

k. Twin wires must never be employed in this class of concealed work.

Wires are said to be "fished" when they are started in at one end of a concealed space and then, so that they may be pulled through, are felt for, or "fished" for, from the other end, with a hooked wire or other contrivance. Such a procedure leaves doubt as to the condition in the space between the entrance and the exit, for the insulation may be scraped, or the wires may rest on pipes. Figure 54 illustrates wires "fished on the loop system." The idea of the rule allowing this method, is to provide for construction in old buildings where it is not possible to conceal the wire by means of such construction as would be employed in new buildings. According to the rule, wire in a flexible conduit may be fished just as the wire alone would be fished, and is threaded-in through concealed spaces so that it makes

the tour, so to speak, of all points where lamps are to be connected. The object of confining this construction to the loop system is to prevent joints being made in concealed spaces. The construction is intended principally for installations in which only a small number of lamps are used.

FIG. 55.-Twin wire. A, Copper wires. B, Rubber insulation

separating and surrounding

copper wires. C, Braided out- FIG. 54.- Flexible Conduit, run on the er covering. Loop System.

"Twin wires" are two wires laid together and surrounded and separated by insulating material, so that the whole forms one piece. The wires are separate conductors and form the two sides of the circuit. (Fig. 55.) Of course any jamming or bruising of this piece is apt to bring the two sides of the circuit into contact and thus form a short circuit.

In concealed spaces more exacting requirements are necessary than with open work, because the construction. cannot be easily examined, any defect is more serious, and there is more temptation to the workmen to slight the work. Compliance with Rule 19 (h) requires noncombustible insulating-tubes to be used where wires pass through walls, wood, etc., and allows only such wire and insulation as that specified for places exposed to moisture.

20. Mouldings:

a. Must never be used in concealed work or in damp places.

b. Must have at least two coats of waterproof paint or be impregnated with a moisture repellent.

c. Must be made of two pieces, a backing and capping, so constructed as to thoroughly encase the wire and maintain a distance of one-half inch between conductors of opposite polarity and afford suitable protection from abrasion.

Moulding is simply a protection against mechanical injury, and in damp places may be even worse than nothing about the wire, since an air-space is an insulator while wet wood is a fair conductor. In dry places, however, moulding guards against mechanical injury to the insulating covering of the wire, and is a sufficient insulator itself. Saturating the moulding with a substance that will prevent the absorption of moisture guards

against the danger of leakage in case of accidental conditions. The moulding is of course an additional danger in case of overheated wires, and though its use is allowed in a limited way it is not thought to afford first-class construction. (Fig. 50.)

21. Special Wiring:

In breweries, packing-houses, stables, dye-houses, paper and pulp mills, or other buildings specially liable to moisture or acid, or other fumes liable to injure the wires or insulation, except where used for pendants, conductors

a. Must be separated at least six inches.

b. Must be provided with an approved waterproof covering.

[Section b. The insulating covering of the wire to be. approved under this section must be solid, at least threesixty-fourths of an inch in thickness and covered with a substantial braid. It must not readily carry fire, must show an insulating resistance of one megohm per mile after two weeks' submersion in water at 70 degrees Fahr., and three days' submersion in lime-water, with a current of 550 volts after three minutes' electrification, and must also withstand a satisfactory test against such chemical compounds or mixtures as it will be liable to be subjected to in the risk under consideration.]

c. Must be carefully put up.

d. Must be supported by glass or porcelain insulators. No switches or fusible cut-outs will be allowed where ex

posed to inflammable gases or dust, or to flyings of combustible material.

e. Must be protected when passing through floors, walls, partitions, timbers, etc., by waterproof, non-combustible, insulating-tubes, such as glass or porcelain.

The requirements stated under Rule 21 are, it will be noticed, simply more exacting than those for work subject to ordinary conditions. Rule 21 (d) meets a special condition that sometimes exists, and is made necessary because the action of switches and the melting of fuses cause an arc of short duration that might ignite gases or finely divided dust. The wire for these special places must stand a more severe test, and the list of approved wires given on page 80 is not necessarily correct for all these special conditions.

22. Interior Conduits :

[The object of a tube or conduit is to facilitate the insertion or extraction of the conductors, to protect them from mechanical injury, and, so far as possible, from moisture. Tubes or conduits are to be considered merely as raceways, and are not to be relied on for insulation between wire and wire, or between the wire and the ground.

The American Circular Loom Company Tube, the brasssheathed and the iron-armoured tubes made by the Interior Conduit and Insulation Company, and the Vulca Tube are approved for the class of work called for in this rule.