{Contributed by H. Y. Margary)

The principal accessories to electric-lighting installations, such as lamps, fuses, and switches, have been already mentioned in the chapter upon the arrangement of wires in a building, but it is necessary that they should now be more fully described.


There are two forms of electric lamp in general use; namely, the Incandescent lamp and the Arc lamp. Among the incandescent lamp group is the more modern Nernst lamp, whose filament requires heating by some exterior means before it glows or "incandesces," and the most recent "Osmium" and "Tantalum" lamps, whose efficiencies of candle-power per unit of electrical power consumed are said to be higher than those obtained by carbon filament lamps.

Incandescent lamps consist essentially of a thin filament of carbon contained within an exhausted glass globe. Various patterns of lamps are manufactured, designed for various pressures and candle-powers, but all on the same principles.

Lamps will usually burn for about 1000 hours if they are of good quality, but if of inferior quality their candle-power will drop, while the power required to light them will increase per c.p. so considerably that it does not pay to use them after the first 700 to 800 hours. Thus an Ediswan or other good make of lamp, of the 16 c.p. type, will emit 17 c.p. for the greater part of its life, and consume electrical energy at the rate of about 4 watts per c.p.; while a lamp of inferior quality will rarely emit as much as 15 c.p. when new, which c.p. will often drop as low as 8, with a consumption of energy starting at 4 watts per c.p. and rising to about 6.5 within the first 600 to 700 hours. Although good incandescent lamps consume energy at the rate of 4 watts per c.p., or 32 watts per 8 c.p., 64 watts per 16 c.p., and 128 watts per 32 c.p., it is usual, for the sake of simplicity, to calculate the size of the cables for such lamps at the rate of 30, 60, and 120 watts respectively.

The ends of the filament are connected to platinum wires, which are fused into glass supports. The ends of the platinum wires are connected to brass studs contained within the brass collar. Two brass pins project from the collar for fixing the lamps to the lamp holders.

Lamps are also made with frosted opal or tinted globes, or with one side of the globe silvered to act as a reflector. The actual light emitted from the filament of a lamp is absorbed by the globes to the extent shown in the following table :-

Clear glass absorbs

10 per cent.

Ground " . .

30 to 50 "

Opal "

50 to 60 "

Arc Lamps

If two pieces of carbon be connected to the two poles of an electric circuit, and the points of the carbons be touched together and slowly drawn apart, a spark of great intensity is formed between them. A contrivance for mechanically performing the operation of touching the carbon points and drawing them apart again forms the essential portion of an arc lamp.

Fig. 164 shows diagrammatically the general arrangement of the mechanism of an arc lamp of the " enclosed " or long-burning type. The whole arrangement is suspended from a porcelain insulator A. The current is fed through the terminal B, B, insulated from the rest of the mechanism. The positive wire leads through the coil E to the split tube D, which holds the carbon G, and is so constructed that it can slide up and down in the tube C. The negative wire is connected to the lower carbon rod H through the support S. The lower carbon is clamped in position by the screw T. The carbons are adjusted so that they touch one another, and when the current is switched on the core E becomes magnetised and attracts the iron cone F, which pulls down the arm K. which is pivoted to the tube C, so that the arm L and the cluten M are drawn upwards. The clutch M grips the carbon G, and draws it away from the carbon H, and so the arc is established. To prevent the coil from moving the core too suddenly, a contrivance known as a dash-pot is used, the action of which is as follows: - When F is drawn downwards, N and O are drawn upwards ; O is a piston fitting closely into the cylinder P. The air can only enter slowly into the space beneath O, so that O can only move slowly, and thus checks any sudden jar. The mechanism is mounted upon a plate Q, which also serves the purpose of a reflector and a holder for the globe R. Arc lamps are usually connected up two or more in series, with a resistance, which has the effect of steadying the arc. The lamp shown must be considered as diagrammatic only, but is based upon modern practice as regards the enclosed arc lamp, which has now practically taken the place of the older "open type" arc lamp. The only exception to this practice is the well-known Foster lamp, in which neither coil nor dash pot is used; but in which the formation and maintenance of the arc is for the purpose of this work substantially the same.

The Nernst lamp is a form of incandescent lamp, the filament or "glower," as it is technically termed, being composed of a thin rod of china-like material containing a large percentage of some alkaline earth, which has the property of glowing with an intense whiteness when heated. This form of filament will not conduct an electric current until its temperature has been raised to a bright redness. The lamp must therefore be fitted with an arrangement for heating the glower until it is capable of conducting a current.

Arc Lamps 201

Fig. 164.

Fig. 164A shows the various parts of a Nernst lamp. A is a china fitting for connecting to an ordinary bayonet socket. Inside A is fixed a small electromagnet B for electrically switching the current from the heating arrangement D to the glower E. The glower and heater are mounted upon a china base F, which is connected to the rest of the mechanism by slipping the open tubes G over the split rods. The heater is composed of a zigzag rod of china, with a spiral of fine platinum wire bedded in its surface. K is a resistance, composed of fine iron wire spirals contained within an exhausted glass bulb, and connected to the two metal plates L, L, which slide between two spring clips M, M. The dotted lines represent the resistance bulb in position. The mechanism of the lamp is protected by means of the cover N, which screws on to the metal thread at O. P shows the glass globe broken away.