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which a joint may be dangerously defective. underwriters furnish lists of the insulating-joints that have been tested and approved.

There is an opportunity for careless work at the tops of fixtures where the "supply-conductors" (Fig. 66) come through the ceiling to be attached to the "fixture wires. These supply-wires

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are brought down close to the gaspipe to which the fixture is attached and should, as Rule 27 (b) requires, be carefully kept away from the part of the gas-pipe system that is connected with the ground. "Shells" (Fig. 66) are ordinarily used, and as these cover up the work there is less inducement for the workmen to do the

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FIG. 66.-A, Gas-pipe. B, Supply-conductors. C, Ceiling. D, Insulat ing-joint. E, Fixture wires. F, Shell. G, Outer Casing of Fixture. H, Splices.

work carefully, even when the shell is large enough to take the wires and splices without crowding.

"Burrs" and "fins" are the sharp edges left when the sheet-metal composing the fixtures is cut. These, if left, will injure the insulating covering of the wire when it is drawn over them.

The upper ends of fixtures are usually "sealed" by wrapping tape (page 89) about them until they are virtually air-tight.

The gas-pipe running down through the fixtures is surrounded by a larger tube or "casing," and between these the electric wires are run. When this space is

too small for the wires to be drawn through easily, the insulation is apt to become abraded.

The testing required in Rule 27 (h) can of course be satisfactorily done only by those familiar with the work. Fixtures are often fastened against a block that is attached to the ceiling. If this "ceiling-block" itself is not of a material suitable for bushings, a regular insulating bushing, such as would be proper through the ceiling, should be used.

28. Arc Lights on Low-potential Circuits:

a. Must be supplied by branch conductors not smaller than No. 12 B. & S. gauge.

b. Must be connected with main conductors only through double-pole cut-outs.

c. Must only be furnished with such resistances or regulators as are enclosed in non-combustible material, such resistances being treated as stoves.

Incandescent lamps must not be used for resistance

devices.

d. Must be supplied with globes and protected as in the case of arc lights on high-potential circuits.

Arc lights are often run from the same circuit that supplies incandescent lamps, by connecting two in series.

between the two wires of the circuit. (Fig. 20.) Each lamp takes from 50 to 55 volts and the two can therefore be operated by the 110 volts pressure of the lowpotential circuit. It has been remarked that with a regular series circuit the dynamo is so regulated that it maintains a constant current. Since the circuit operating incandescent lamps has a constant pressure, it is necessary to have with these arc lamps on incandescent circuits, resistances that will be cut in or cut out automatically, as the resistance of the arc lamps decreases or rises with the feeding and burning of the carbons. From remarks on page 90 it will be understood why fuses cannot be used with a constant-current circuit; but they should be used with a constant-pressure circuit, and each circuit of arc lamps on an incandescent circuit should have double-pole cut-outs with fuses, just as incandescent lamps or motors would have them.

The resistances used with arc lamps heat as any resistance does when a current is forced through it, and when the lamps get badly out of order the resistances sometimes become greatly overheated. As the rules state, it is necessary to install these resistances. only in such places and in such ways as would be proper if they were small stoves.

Incandescent lamps have a definite amount of resistance in their filaments and they are made to stand considerable current for their size, so that they make

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convenient resistance devices. They are poor for this purpose, however, for if any of the lamps burn out, it may upset the balance and cause excessive current, as in multiple-series or series-multiple systems of lighting. (Page 102.)

29. Electric Gas-lighting:

Where electric gas-lighting is to be used on the same fixture with the electric light

a. No part of the gas-piping or fixture shall be in electrical connection with the gas-lighting circuit.

b. The wires used with the fixtures must have a noninflammable insulation, or, where concealed between the pipe and shell of the fixture, the insulation must be such as required for fixture wiring for the electric light.

c. The whole installations must test free from "grounds." d. The two installations must test perfectly free from connection with each other.

The lighting of gas by electricity requires only battery power of low electrical pressure, and neither the insulation on the wire nor the method of construction need be nearly so good for satisfactory operation as electric-light wiring must be. Often the gas-pipes are made to serve as one wire of the gas-lighting circuit. Connection between the gas-lighting system and the electric-light system, exposes the inferior insulation to the comparatively high pressure of the light system, and such cross-connections can cause serious damage. Accordingly where the two systems are used together

on the same fixture, it is required that the standard of the gas-lighting wiring be raised virtually to that required for the lighting wiring.

30. Sockets:

a. No portion of the lamp-socket exposed to contact with outside objects must be allowed to come into electrical contact with either of the conductors.

b. In rooms where inflammable gases may exist, or where the atmosphere is damp, the incandescent lamp and socket should be enclosed in a vapour-tight globe.

The "socket" (Figs. 67 and 68) is the brass receptacle into which the lamp itself is fastened. One terminal of the lamp filament is attached to the brass band

FIG. 67.-Key- FIG. 68. Key less Socket. Socket.

about the lamp base, and the other terminal of the filament is attached to the smaller brass piece coming out of the centre of the lamp base. The wires are brought into the socket and attached to terminals so arranged that when the lamp is fastened in the socket, each end of the filament is in electrical connection with one of the two wires of the circuit.

The space within the socket is small and though there is room for the conductors to enter and to be attached to the socket terminals, carelessness in making the con

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