kept apart, but they should have an insulation as good as that of any other parts of the wiring. Section "a' of Rule 21 requires that the wires shall be kept six inches apart in places where they are exposed to the action of chemicals. This is equivalent to a decision that it is as difficult to maintain insulation on a low potential system, where the wires are thus exposed, as it is to maintain insulation on a high potential system, under ordinary conditions. In reality the difficulty is greater; and in some cases it is almost impossible to maintain a high insulation with any kind of material and construction.

Section "b" calls for the highest grade of insulated wire. The life of the insulation on a wire when exposed to chemical action is usually short. The.best insulation that the market affords is none too good for this class of work and the safe thing to do (and the cheapest thing in the long run) is to buy the very best wire that can be bought for use in places such as are described in this rule. Even if we do this we cannot be sure of our results unless we select a make of wire which has a record for withstanding the action of the particular chemicals to which it is liable to be exposed.

The simple rule laid down in section "c" is the most important of any to be observed. Where wires are run in the air and are supported upon insulators, the only places where the current can leak from one wire to another or from a wire to the ground is at the points of support. If a wire is carelessly put up, it is liable to be injured at these very points. If the insulation becomes broken, then no matter how good it was

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is no better than a bare wire. If - in a wire or if is tied too tightly to anent strain is placed upon the rubits life may be reduced to a small would be if put up with judgment ill and care can secure work that ation and will insure safety in case ng of the wire is injured.

truction called for in section "d" de in dry places and it is still more hall be employed in wet ones. In cipal virtue of our glass and porcefire proof. In damp places howin places where wires are exposed we need glass or porcelain to help

1, we should notice that most of the his country in the past and much of on the market, is almost worthless as concerned. Only a few years ago to use glass exclusively for use in the like, owing to the inferior qualwhich was then used for making or porcelain is so porous that it will e a sponge, so that all the insulation the thin glaze on the surface. Since ed on the bottom, a porous knob on ome thoroughly saturated with moist

a check in the glaze, the knob prace an insulator at all. When porcelain ork it should be thoroughly vitrified,

and any porcelain which is not thoroughly vitrified should be condemned. We have seen that when a circuit carrying a current is opened, an arc is formed. This is just what we get when a circuit is opened by the opening of a switch or the blowing of a fuse. The danger of allowing a spark to be made in the presence of inflammable gases or of fine dust is too well understood by all insurance men to need any comment. By making an installation in accordance with the spirit of Rule 21 we can not only furnish light in places such as are described in section "d" (without creating a hazard), but a light which is safer than any other kind of illuminant.

The code requires that all wires, whether of high or low potential systems, shall be protected wherever they pass through walls by insulating tubes. For high potential systems and for the class of work referred to in Rule 21, the tubes must be of glass or porcelain or some other incombustible material. Rule 18, which we have considered in a previous chapter, allows the use of tubes of other materials on low potential systems, in places where wires are not liable to be exposed to moisture; but as almost any wall or partition is liable to be a place where wires are exposed to moisture, and, as tubes of porcelain are at present quite inexpensive, and nearly or quite as cheap as the other bushings referred to in Rule 18 and the definitions explaining that rule, it can hardly be considered good practice to run wires of any kind through walls or partitions without protecting them in the manner described in section "c" of Rule 21.

CHAPTER XI.

CLASS C, LOW POTENTIAL SYSTEMS.

PART V.

TEXT OF THE CODE COVERED BY THIS CHAPTER. 22. INTERIOR CONDUITS*:-(See Definitions.) a. Must be continuous from one junction box or another, or to fixtures, and must be of material that will resist the fusion of the wire or wires they contain without igniting the conduit. b. Must not be of such material or construction that the insulation of the conductors will ultimately be injured or destroyed by the elements of the composition. c. Must be first installed as a complete conduit system, without conductors, strings, or anything for the purpose of drawing in the conductors, and the conductors then to be pushed or fished in. The conductors must not be placed in position until all mechanical work on the building has been, as far as possible, completed. d. Must not be so placed as to be subject to mechanical injury by saws, chisels or nails. e. Must not be supplied with a twin conductor, or two separate conductors, in a single tube. (See Rule 22.) f. Must have all ends closed with good adhesive material, either at junction boxes or elsewhere, whether such ends are concealed or exposed. Joints must be made airtight and moisture-proof. g. Conduits must extend at least one inch beyond the finished surface of walls or ceilings until the

*The object of a tube or conduit is to facilitate the insertion or extraction of the conductors, to protect them from mechanical injury, and, as 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.

NOTE. The use of two Standard wires, either separate or twin conductor, in a straight conduit installation is approved in the iron-armored conduit of the Interior Conduit and Installation Company, but not in any of the other approved conduits. (See Rule 22, e.)

mortar or other similar material be entirely dry, when the projection may be reduced to half an inch.

RULE 22.

DEFINITIONS. INTERIOR CONDUITS:-The brasssheathed and the iron-armored tubes made by the Interior Conduit and Insulation Company, the American Circular Loom Co. tube and the Vulca tube are approved for the class of work called for in this rule.

Interior Conduits.-We have already called special attention to the fact that in order to have the best kind of an installation, it is necessary to have the wires at all times accessible. Accessibility may be obtained in three ways: First, by running wires upon knobs or cleats, the wires being either in sight upon walls or ceilings, or in the hollow spaces, as in attics, unfinished rooms, ventilating shafts, etc., or in runways specially provided for the purpose. Second, by running the wires in wooden mouldings so constructed that we can remove the covering or "capping" and inspect the wires or remove and replace them in case of injury to the insulation. These two methods suffice to secure good insulation, but they do not, except in special cases, admit of construction which will allow the wires to be concealed from view and at the same time protected against mechanical injury. The third method consists of running the wires in a pipe or conduit so constructed that the wires can be readily withdrawn and other wires drawn in, if at any time the insulating covering of the wires is injured. This system, if properly installed, allows us to withdraw the wires for inspection and to readily insert them again. The object of the interior conduit is briefly and very clearly set forth in the note given above in the text of the code.