nections is apt to leave one or more of the fine wires making up the conductor projecting, so that it is free to come into contact with the socket.

There is considerable likelihood of short circuits in sockets, and though the fuses allow an excessive current to last only momentarily, if there were inflammable gas present even the very short flash would be sufficient to ignite Where the atmosphere is

e

С

FIG. 69.-Vapourtight Globe to cover Lamp and Socket.

the gas.

damp, short circuits in the sockets are apt to be numerous and though there is a probability that the fuses will take care of these, it is poor practice to depend upon safety-devices when the trouble may be prevented. "Vapour

tight globes" (Fig. 69) cover both the lamp and the socket and are an effectIt is well to use them when fine dust accumulates about the socket and the attached conductors.

ual protection.

31. Flexible Cord:

a. Must be made of conductors, each surrounded with a moisture-proof and a non-inflammable layer, and further insulated from each other by a mechanical separator of carbonizable material. Each of these conductors must be composed of several strands.

b. Must not sustain more than one light not exceeding 50 candle-power.

c. Must not be used except for pendants, wiring of fixtures, and portable lamps or motors.

d. Must not be used in show windows.

e. Must be protected by insulating bushings where the cord enters the socket. The ends of the cord must be taped to prevent fraying of the covering.

f. Must be so suspended that the entire weight of the socket and lamp will be borne by knots under the bushing in the socket, and above the point where the cord comes through the ceiling-block or rosette, in order that the strain may be taken from the joints and binding screws.

g. Must be equipped with keyless sockets as far as practicable, and be controlled by wall switches.

"Flexible cord" is the name given to the species of twin wire that is formed by twisting two stranded conductors covered with insulation, about each other so that they make one cord, or piece. It is much used for pendant lamps and for portable lamps and motors, but its convenience tempts one to use it in many places for which it is not at all suitable. Since the conductors forming the two sides of the circuit are twisted close together, it takes very little abrasion or injury to the insulating covering to make a connection between these two conductors and thus form a short circuit. In addition to the usual dangers of short circuits, the silk or cotton braid covering the wires may take fire from the flash.

The rules require that each conductor of flexible cord be made up of several strands, because if a solid conductor be moved about as if it were perfectly flexible, it will sooner or later break off and cause trouble. Each conductor is to be surrounded by a non-inflammable layer to preserve the insulation in case of overheating, and there is also to be a moisture-proof layer of insulation so that accidental moistening will not cause a short circuit. The wires being also separated by a carbonizable material, if bad leakage does occur, this carbonizable separator will become a good conductor, will help the passage of a heavy current, and thus quickly cause the melting of a fuse and the stopping of further current. The two insulating layers are required so that liability of leakage between the wires may be reduced to a minimum; but when leakage does occur it is thought better to have it cause a short circuit immediately and melt the fuse.

Flexible cord is not intended to sustain weights, and the rules limit the weight that may be suspended, by allowing on one cord but one incandescent lamp; and this must not exceed in size a 50 candle-power lamp.

Flexible cord is not allowed in show windows even for pendants. There is always liability of its shortcircuiting and thus causing at least a momentary blaze and perhaps a scattering of particles of molten

metal. With the inflammable material usually present in show windows, these short circuits that might amount to little elsewhere may here quickly start a fire.

When

Where the cord enters the socket there is gradual wear of the insulation by abrasion unless the hole is bushed with an "insulating bushing" (Fig. 70). the insulation is scraped from the ends of the conductors so that they may be connected to the terminals in the socket, the

B

Insulating
Bushing

for Socket.

light ends are inflam-
mable and would quickly catch
fire from a small arc in the socket.

FIG. 71.-Ceiling-block It is accordingly required that the

or Rosette. A, Connections for Supplyconductors. B, Fuses.

C, Hole for flexible

cord, against the sides

of which the knot bears. D, One of the

cord.

ends of the conductors be wound with tape to bind down the loose covering.

If a cord pull directly from the Binding-screws for binding-screws holding it in the socket or in the ceiling-block, or "rosette" (Fig. 71), the strands are apt to pull out from under the screws and either leave too few strands to carry the current safely, or else leave loose ones to fly against the other wire of the circuit and cause an The holes in the socket bushing and in the

arc.

rosette are little larger than enough to allow the cord to slide through them, and by tying a knot in the cord after it has passed through the hole, the knot comes against the walls of the hole and relieves the binding screws of any strain.

Every socket that has a key for turning the light on and off, has virtually a switch within it that draws a slight arc when it operates and that is liable to get out of order. It is well to get rid of these small switches as much as possible and so reduce the chances of trouble. With "keyless sockets" (Fig. 67) the connections are permanently made, and several lights are turned on and off by one switch on the wall or in a switch closet. This switch is larger than the socket switches, and as there is not so limited a space in which the parts must be assembled, it is less likely to get out of order.

32. Decorative Series Lamps:

Incandescent lamps run in series circuits shall not be used for decorative purposes inside of buildings.

Decorative "series" lamps would never be operated on a pure series circuit, but would be operated on a multiple-series system, strings of lamps being connected-in between the wires of a constant-pressure circuit. (Fig. 45.) Rule 14 (c) prohibits any system of series-multiple or multiple-series lighting for in