This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
20. The incandescent lamp, Fig. 9, is provided with a composite base, made to fit in a corresponding socket connected with the house wiring. The filament F is made of a thread of carbon, which material has a very high resistance. When the filament is enclosed in a vessel from which the air has been removed, and a current is passed through it, it becomes incandescent and gives out light. Connections are made from one end of the filament to the outside metallic screw sheathing s, and from the other end to the brass plug o at the bottom. The socket, Fig. 10, has a screw shell s inside, large enough to receive the base of the lamp, and at its center an insulated tongue t, which presses against the plug on the lamp when the latter is screwed in. The wires supplying current are connected, one to the tongue t by means of the screw a at its lower end, the other to a similar screw (not shown) on the lower contact strip c. When the key k is turned into the vertical position, as drawn, the bridge b completes the circuit between the strip c and the sheathing s, pressing against a contact piece on the lower side of the porcelain plate p, which is in electrical connection with s. When the key is turned through 90°, the contact between c and s is broken, and the light is extinguished. The socket may be suspended from a flexible cord (A, Fig. 6) attached to a ceiling rosette, or fixed to a bracket or electrolier.
21. A rosette pendant is shown in Fig. 11. The leads are brought to the screws s, s1 on the insulating base b, and the fixture wires are also secured to the same contact-blocks, and pass down inside the tube t to the lamp. A casing c fits up against the base, the opening at the lower end being larger than the tube, and closed by a washer w held by a collar k. The pendant is thus free to swing a certain amount, being supported on a hook h passing through a loop l on the tube. When flexible-cord suspension is used, a knot should be made in the cord inside the rosette, and also, if practicable, in the lamp socket, to prevent the weight coming on the connections.
22. Combination fixtures for gas and electric light are frequently installed. The fixtures must in this case be insulated from the gas piping of the building, a joint being used, such as is shown in Fig. 12, in which the two metallic unions a and b are separated from each other by an insulating cement c, and the whole joint further protected by a covering d of hard rubber or similar material. No soft rubber must be used in the construction. At the point where the wires approach the fixture, they must be kept clear of the grounded gas pipe, and the outer shell at the point of entrance must be large enough to allow this. In Fig. 13 the arrangement of wires is shown with relation to the piping for a chandelier. The gas pipe g is carried across between the floorbeams, and the insulating joint i is placed as already indicated, to form the connection. The wires are brought over in independent insulating conduits c, c', as described in detail later, and inserted through the tube t surrounding the gas pipe g. There must be between these at least a quarter of an inch clear space, to conform with the insurance regulations. After the wires are in place, the hood h is pushed up against the ceiling, and secured by means of the collar k furnished with a setscrew s. It is necessary to examine carefully all edges of the metal shell, that any rough parts near the wires be smoothed down. The hood when in position must clear the wire, and not press it against the insulating joint; and, to guard against the entrance of moisture, the upper end of the fixture should be sealed.