tubes, in addition to non-combustible tubes, thus far approved for dry places are the kinds mentioned under Rule 18 (b).

Rule 18 (c) is made to insure proper insulation of the wiring from the ground. (Page 59.) The wire, if not rigidly fastened so that it ducting material, may, by being struck or by sagging, come into contact with this material. The aim of the rule is to keep the wire away from conducting material of any kind, but the greatest danger comes from pipes because they are directly connected with the earth. Construction similar to that shown in Figure 48 will insure the separation. Conductors that are not connected directly with the earth, such as a nail driven into a beam, appear sometimes to be harmless, but it should be remembered that whereas a slight contact between the wire and the beam itself might result in virtually no leakage, a driven nail presents a comparatively large surface to the wood, and if the wood were at all moist, contact with the nail might result in considerable leakage and cause dangerous sparking.

will be held away from con

Construction is allowed with low-potential systems that is not allowed with high-potential systems, and Rule 18 (d) is made so that there can be no possibility of contact between the two.

An air-space is required where wires cross pipes in damp places, because there would be danger of leakage

I

over the surface of any material between the two, if this material were damp. If wires are run under pipes on which moisture gathers, the moisture may cause a cross-connection between the pipe and the wires. (Fig. 49.)

All joints are to be soldered, as they are with highpotential circuits. (Page 84.)

SPECIAL RULES.

19. Wiring not Encased in Moulding or Approved Conduit:

a. Must be supported wholly on non-combustible insulators, constructed so as to prevent the insulating coverings of the wire from coming in contact with other substances than the insulating supports.

b. Must be so arranged that wires of opposite polarity, with a difference of potential of 150 volts or less, will be kept apart at least two and one-half inches.

c. Must have the above distance increased proportionately where a higher voltage is used.

d. Must not be laid in plaster, cement, or similar finish. e. Must never be fastened with staples.

"Moulding" is made in two pieces so as wholly to encase the wire, and thus acts as a protection to the insulating covering. (Fig. 50.)

"Conduit" is tubing that serves as a raceway or channel for wires, the building being piped with it as if for water or gas. The wire is then run inside, so that

the whole wiring installation is isolated and protected. (Pages 51, 122.)

The two wires of a circuit are wires of " ' opposite polarity." The convention has been adopted of calling the side of the dynamo from which the current is supposed to start, the positive or + pole, and the side to which the current returns, the negative or - pole. The wires leading from the dynamo are thus spoken of

as being of opposite polarity, one being the positive pole of the circuit and the other the negative pole.

It will be noticed that there are only three approved ways of running wires: in moulding, in conduit, and wholly on non-combustible insulators. The intention of Rule 19 (a) is to prevent possibility of leaks, and also to have the wire in contact only with non-combustible material so that no harm will come in case the wire be excessively heated by a heavy current. While

everything is perfectly dry there is no danger of leakage, and it was formerly the practice to fasten the wire directly against wooden ceilings; but it is not known when conditions may change by the spilling of water, or the leaking of roofs, and moreover the allowing of a poorer construction in some cases, leads to the adoption of this construction in other cases when it is not at all suitable. To use the construction everywhere that is everywhere good, costs but little more and is much safer and more satisfactory.

FIG. 51. Wires grounded through Moist Plaster and Pipe, thus forming a Cross-connection between the Two Sides of the Circuit.

A very short air-space is sufficient insulation with such pressures as are used in practice, but the distances between wires, specified in Rule 19 (6), are required because wires that become slightly loose are apt to come together unless they were originally some distance apart. Moreover, leakage may take place over damp surfaces, and it is wiser not to make the distances too short.

Rule 19 (a) covers the construction mentioned in Rule 19 (d), but it is brought out specifically in the latter rule to give greater prominence, and because it might be thought that plaster and cement are noncombustible insulating materials. On the contrary, they are fair conductors when moist, and moreover they act on the insulating covering of the wire, finally destroying the insulating properties. Even though no harm were done at the point where the leakage takes place in the plaster, such leakage might "ground" one side of the circuit and thus make possible at another point, as at a gas-fixture, a short circuit that would burn through the pipe Metal and ignite the gas. (Fig. 51.)

FIG. 52.

Staple.

The ordinary staple (Fig. 52) is of course a direct violation of Rule 19 (a), but staples are made that are insulated and that keep the wire out of contact with other substances. (Fig. 53.) These are also prohibited, however, for in fastening the wire with them they are driven into the wood and are liable to injure the insulating covering of the wire.

IN UNFINISHED LOFTS, BETWEEN FLOOR AND CEILINGS, IN PARTITIONS AND OTHER CONCEALED PLACES.

f. Must have at least one inch clear air-space surrounding them.

g. Must be at least ten inches apart when possible, and should be run singly on separate timbers or studding.