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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.
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). When the insulation is scraped from the ends of the conductors so that they may be con- 1 nected to the terminals in the socket, the
outside braid is apt to Fio. 70. — become frayed. These 5£J« light ends are inflam- for Socket, mable and would quickly catch fire from a small arc in the socket. It is accordingly required that the ends of the conductors be wound with tape to bind down the loose covering.
If a cord pull directly from the 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 arc. The holes in the socket bushing and in the
Fio. 71. — Ceiling-block 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 Binding-screws for cord.
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 ini
candescent lamps, but the rule applies only to circuits of 300 volts or over. The reasons for Rule 14 (c) will apply with more or less force to all series-multiple or multiple-series lighting, and the danger is considerably increased with decorative lamps because they are usually surrounded by inflammable material.
ALTERNATING SYSTEMS. — CONVERTERS OR TRANSFORMERS.
a. Must not be placed inside of any building, except the Central Station, unless by special permission of the Underwriters having jurisdiction.
b. Must not be placed in any but metallic or other non-combustible cases.
c. Must not be attached to the outside walls of buildings, unless separated therefrom by substantial insulating supports.
34. In those Cases where it may not be possible to exclude
the Converters and Primary Wires entirely from the
Converters must be located at a point as near as possible to that at which the primary wires enter the building, and must be placed in a room or vault constructed of, or lined with, fire-resisting material, and used only for the purpose. They must be effectually insulated from the ground, and the room in which they are placed be practically air-tight,
except that it shall be thoroughly ventilated to the out-door air, if possible through a chimney or flue.
Any explanation of alternating currents and of the laws regulating their action requires so technical a discussion that a simple statement of important facts will probably leave clearer ideas than would an attempt at an exposition.
With the alternating system the dynamo ordinarily generates a pressure of 1000 or 2000 volts. "Transformers" or "converters" are then used to change this pressure, at the place where the current is to be used, into a lower pressure of 50 or 100 volts. The pressure is high before the transformation is made and the current comparatively small; after the transformation the pressure is low and the current comparatively large. The available amount of energy is but little changed by this transformation or conversion; the current multiplied by the pressure (the number of watts) is virtually the same before as after.
This transformation is possible with alternating currents because of the principle called "induction." Suppose (Fig. 72) a coil of wire B to be lying near, but not touching, another coil A in which a current may be started either in the direction of the arrows C, C, or in the direction of the arrows B, B. Without discussing the hypotheses made to account for the