This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
The drawing in the left-hand top corner of the figure shows diagrammatically the manner in which the lamp is connected up. The current enters at the positive terminal, along the contact spring curvature a, through the heater b and back to the negative terminal. The glower c, which is actually very close to the heater, becomes heated, and when at red heat the current finds an easier passage through the glower than through the heater, with the result that the current passes through the electromagnet d, through the resistance e, through the glower f, and back to the negative terminal. The current, on passing round the electromagnet, energises its core, drawing the armature towards it, and so disconnecting the heater.
The fittings used for connecting lamps to a circuit are called lamp holders, the general types of which are illustrated at A and B, Fig. 165, - A being used for pendants and B for lights attached to walls or ceilings. This latter type is termed the batten holder.
The various portions of a pendant lamp holder are shown at C. The porcelain interior is inserted in the socket, and the bayonet points are kept in a definite position relatively to the hooks or bayonet sockets O by means of the grooves G, G, which slip over the projections P, P. The cap S is placed over the porcelain interior, and is held there by means of the screw ring R, which is slipped up from the bottom of the socket and catches the flange F. When a shade is used the portion of the socket marked T has a thread worked upon it when the shade is placed in position, and a shade ring W is screwed up from beneath it.
The lamp is connected by pushing the two pins on the collar up the slots on either side of the socket and twisting the lamp, so that the pins catch on the hooks O, where they remain owing to the downward pressure of the springs in the contacts (see D, Fig. 165).
The wires are connected by stripping off the insulation at the ends and placing them in the holes H, H, where they are fixed by the two screws. The contacts press against the two brass studs in the lamp collar.
Wall Sockets and Plugs are contrivances used for connecting up table or standard lamps, and are of three kinds - the two-pin, bayonetsocket, and concentric.
Two-pin Sockets consist of a block of porcelain A, Fig. 166, containing terminals T for fixing the leads, and a porcelain cover B. The terminals are made with long brass split sockets, as shown at S. The block A is fixed to the wire casing, or to the skirting, to wood plugs or fixing blocks by means of screws through the holes H. The cover is fixed by means of a screw passing through the hole in the centre, and through the hole J in the block A. The plug consists of two split pins P, P, fixed to an ebonite disc, which is attached to a porcelain cover by means of two screws. The flexible wires are fixed to terminals as shown, and are gripped firmly by the little ridges R formed on the inside of the cover. No switch is used with wall plugs of this form, as it is not easy to short-circuit them, the small brass sockets being well protected by porcelain. The act of placing the plug into the socket completes the circuits through the lamp attached to the plug.
Bayottet Sockets are usually made of batten lamp holders with a brass or porcelain cover, as shown in Fig. 167. The plugs are similar to lamp collars with an insulated handle instead of a glass bulb, and are inserted in the same way as lamps. A switch should be used in connection with these sockets, as the bayonets are not well enough protected to prevent a short circuit. Fig. 168 shows the concentric type of wall plug and socket. The method of connecting this fitting is shown in the view of the interior. One wire is connected to the terminal T, and the current passes from the terminal through the fuse F to the ring R. The other wire is connected to the terminal S, which is fixed to a metal plate extending to the centre of the socket. A porcelain case protects the terminals from short circuits. The flexible wires of the plug are connected to the two pieces of brass A and B, and electric contact is made with the socket by the pressure of A upon the brass pin at the centre of the socket, and the pressure of B upon the ring R.
Wall plugs are made in various sizes to carry from 5 to 20 amperes.
Most Fire Insurance Offices prohibit the use of wall plugs with fuses in them, because they involve a certain risk from fire, as they are usually fitted to skirtings or other convenient woodwork.
These are used for connecting table lamps or other fittings to the nearest lamp holder. Their construction is precisely the same as that of the plugs used with bayonet wall sockets. Fig. 167 shows a sketch of an adapter.
Ceiling Roses are contrivances for making an efficient connection between the cables and the flexible wires of pendant lights. Fig. 169 shows a very good pattern of ceiling rose. The two terminals are separated by a thick wall of porcelain, which obviates any danger of the formation of an arc. The peculiar formation of this wall is such that the flexible wires may be tightly gripped, so that no pull comes upon the ends attached to the terminals. Ceiling roses are usually fixed by screws to circular wooden roses set upon the casing, and the wooden roses are holed for the reception of the cables, and are fixed with screws to the ceilings joists or to wooden carriers nailed between the joists. Porcelain covers are screwed over the base to protect the terminals and to give a neat appearance.