the lamp is near inflammable material, is a netting with a mesh about 1 inches on a side. In case the globe breaks, the netting will hold all parts in place until a new globe can be provided. (Fig. 40.)

"Spark-arresters" are made in many forms, but they are all designed, as the name indicates, to prevent the small pieces of incandescent material thrown off by the arc from escaping and perhaps setting fire to inflammable material near by. A common form is shown in Figure 41. It consists of a very fine wire screen which fits tight round the globe and encloses the space above. Another form is a flat disk that fits over the top of the globe and has holes for the carbons to pass through. It is now becoming common for indoor lamps to be made in the form shown in Figure 42. The arc is here entirely enclosed by an extension above the globe.

Carbons are usually "plated" because copper is a far better conductor than carbon, and a thin layer of copper on the carbon reduces the resistance and so insures a better contact with the clamps, and thus lessens the waste that comes from driving a current through resistance. This thin layer of copper sometimes flies off in the shape of incandescent particles, so that plain carbons are preferred where inflammable material is near.

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Hanger-boards" are of many forms, but they all

have virtually the same characteristics.

(Figs. 43, 44.)

They are screwed to the ceiling and have conveniences for hanging the lamp so that it may be simply un

C, Handle for turning current on and off. BB, Connections for wires to lamp. AA, Connections for line wires.

hooked or disconnected without disturbing the permanent wiring of the circuit. On the board is mounted a switch similar to the arc service-switch, by which the

C

FIG. 44. Hanger-board.

current may be cut off from the lamp and be made to pass only through the connections on the board. This makes it easy at any time to remove a lamp for repairs

without shutting off the current from any other lamp. The switch must meet the requirements for the serviceswitch. Loose connections and imperfect switches cause heating and arcing, and it is necessary that the hanger-board be non-combustible so that nothing will be set on fire in case of defects.

These troubles may be avoided by hanging the lamp direct from insulated supports and bringing the wires to the lamp without going first to a board, but in this case there is no switch to cut off the current from the lamp, and lamps cannot be taken down for repairs with the same facility.

The greatest danger from the arc lamp is just such danger as would come from any centre of great heat, and it will be noted that the rules have for their object the complete isolation of the source of heat. Inflammable material hanging near will, of course, be in danger of catching fire, and the hot particles thrown off by the arc will be apt to burn material beneath if precautions are not taken. Tight globes, closed at the bottom, will catch most of the sparks, but many small ones are carried up by the heated air, and the sparkarresters are necessary to prevent any of these from escaping. Broken globes are an approach to no globes at all, and may allow large sparks to drop from the lamp. The wire netting thus helps to keep the arc entirely enclosed.

14. Incandescent Lamps in Series Circuits having a Maximum Potential of 300 Volts or over:

a. Must be governed by the same rules as for arc-lights, and each series lamp provided with an approved hand springswitch and automatic cut-out.

b. Must have each lamp suspended from a hanger-board by means of a rigid tube.

c. No electro-magnetic device for switches and no system of multiple-series or series-multiple lighting will be approved.

d. Under no circumstances can series lamps be attached to gas-fixtures.

Incandescent lamps that are made to operate with a comparatively large current are sometimes connected in series circuits. Except that they do not throw off sparks as arc lamps do, there are the same dangers as with other series circuits and the same precautions are to be taken.

The "hand spring-switch" is virtually the same thing as the arc service-switch. It must act in the same way since it is used on a series circuit; that is, it must be made first to form a by-path round the lamp and then to break the connection to the lamp.

The term "automatic cut-out," when used in this connection, means a switch of some kind that will by itself shunt the current round through a by-path when the lamp becomes defective.

The "hanger-board" used with the series incandescent

lamp is much the same as that used for arc-lamps, but it is required that the lamp be hung by means of a rigid tube, because incandescent lamps are likely to be moved about and the insulating covering on the conductors would become abraded. This is a more serious matter with the high-potential series circuits than it is with low-potential multiple circuits, not only because of the higher potential, but because if the conductor in a series. circuit were to break, a long and destructive arc would be formed.

A

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Electro-magnetic devices" for switches are devices that depend for their action upon magnets that become stronger or weaker according to the amount of current that passes round the iron core. The magnetic force works a mechanism that operates the switch. They are objected to because a slight sticking of the movable parts prevents satisfactory working.

A "multiple-series system" is one in which lamps are in different series and these series connected in multiple. (Fig. 45.) There is a constant pressure between the