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CH. XIII] WIRING FOR ALTERNATING CURRENT 513
to put it near the supply intake, especially if that is at a considerable distance from the lantern or other projection apparatus, then in case of a short circuit in working about the lamp or table switch, an excessive current could not flow, and there would be much less danger from fire or the burning out of fuses. (See also § 708).
§ 708. Wiring when the arc lamp is far from the supply.—
When the supply is at a considerable distance from the arc lamp the flexible wire connection is sometimes used for temporary work, but is not suitable for permanent installation.
Instead of a conduit, well insulated wires are sometimes used from the general supply box to the neighborhood of the arc lamp. The wires must be secured by porcelain or other non-conducting supports every meter (3 or 4 feet) which will separate them from the wall i to 2 cm. (>£ in.) and from each other 5 to 7 cm. (2^" in.) and hold them in place. Where the wires pass through partitions, each wire should have its own porcelain tube so that is does not come in contact with the partition. The safe rule in wiring is to treat the rubber covered wires as if they were naked. At the end it is desirable to have a metal box for the special fuse block and switch. An attachment fixture is also very convenient (fig. 270).
For the position of the rheostat, etc., see § 707.
§ 709. Wiring an arc lamp for large currents. — Arc lamps for opaque projection (Ch. VII) and for moving pictures (Ch. XI) require large amperages, and frequently the lamps become very hot, especially if the lamp-house is not large and well ventilated. For lamps requiring the large currents it is best to use flexible cables of higher capacity than is needed outside the lamp-house. The wire should also be insulated with some fire-proof material like woven asbestos.
The ordinary, rubber insulation will answer for low amperages especially when the lamp-house is well ventilated. An excellent wiring material is the flexible cord used for heating apparatus. This has rubber insulation, and also woven asbestos, and the outside is covered with woven cotton to protect the asbestos. Of course a flexible cord of the proper carrying capacity should be selected.