Since an arrangement like that just described would be very costly for a large number of lamps in an intricate building, a compromise between the two systems is generally arrived at, by which, from the distributing slate slabs, circuits radiate to a uumber of lamps, each taking not more than 6 of an ampere. For these lamps, joints are allowed as in the "tree" system, but cutouts only upon the distributing slabs. This means that the number of joints is kept down considerably, and the cut-outs readily found, especially if each pair (for in such a system one would be upon each wire) is inscribed with the number or description of the lamps they protect. All wires, except flexible pendants and those supplying portable lamps, should be protected by being laid in hard-wood casing, consisting of a strip of wood having chased in it two grooves, according to the size of the wires to occupy them, and having, when the wires are placed in position, a thin strip of wood screwed on the face of it to form a cover. In damp positions, as in cellars, the casing should be varnished inside and out. back and front, with at least one coat of shellac A substitute for wood casing has recently come into vogue, and consists of tubing formed of paper, rendered waterproof with bitumen or similar substance. Wrought iron is often employed in works where the treat-ment is of the ronghest kind, and the moisture excessiv

Fig 634   Diagram of the Distributing System of Wiring

Fig 634 - Diagram of the Distributing System of Wiring.

Fig 635   Section of Wood Case and Lid for Electric Wires.

Fig 635 - Section of Wood Case and Lid for Electric Wires.

We have so far considered only incandescent lighting, but lighting by "are" lamps is another form, and of very great value. In describing the breaking of a circuit with a switch just now. whereby a spark was created, it would have been quite correct to have termed such a spark an arc; any space over which electricity passes is. in electrical parlance, an are.

With incandescent lighting, the electrical energy is, as has been shown, consumed in passing through a filament of carbon; but similar energy can be consumed in "arcing" between two points. If a piece of carbon is held in each hand, each piece being connected by a wire to one of the mains of the circuit, and these two pieces of carbon are brought into contact, current will begin to How, and on drawing them slightly apart the electricity will be able to maintain an arc across the space dividing them, and, while doing so, will render the points incandescent and so give light.

There is no other arrangement known, which, for the same amount of energy, will give so much heat as the electric arc, and it is to this that the high state of incandescence is due. It has been seen that 6O watts give, with an incandescent lamp, about 16 candle-powers, but as two arc-lamps, taking (together with the resistance) 100 volts and 10 amperes, or a total of 1000 watts, will give about one thousand candle-powers each, their great comparative efficiency can be at once appreciated.

The carbon points, not being in vacuum (as are incandescent filaments), are slowly consumed, and the pieces of carbon must therefore be gradually fed together, to keep the distance between them constant. Carbon is used in preference to any other material, owing to the high state of incandescence to which it can be raised, and to its lasting properties. The arc has the peculiar action of forming the end of the positive carbon into the shape of a small cup, while the end of the negative carbon is brought to a slight point, which will approximately fit into the positive cup.

The space between the carbon points must be regulated according to the current of electricity, otherwise a white and steady light will not be obtained. With an arc of ten amperes, which is a very usual quantity, the distance apart of the two carbons to obtain this white light should be about one-eighth of an inch; but ten amperes passing through an eighth-of-an-inch arc only require about 45 volts, so if we were to put such an arc on a 100-volt circuit, the potential would be too great, and more than 10 amperes would pass. To prevent this, two such arcs are used in series, that is to say, the current from one main has to pass through one arc and then through the other, before it is allowed to reach the other main. This gives a total of 90 volts absorbed out of the 100 provided by the circuit. The balance of 10 volts is therefore consumed by a resistance placed anywhere within this circuit.

Fig. 636   An Arc Lamp

Fig. 636 - An Arc Lamp

This arc-lamp resistance usually consists of a length of iron wire, wound into the shape of spiral springs so as to occupy as little space as possible, the springs being inclosed in a cast-iron box, as shown in Fig. 637. A resistance, like everything absorbing electrical energy, becomes hot when in use, and is therefore mounted on a slab of slate, and packed out from the wall to which it may be fixed, so as to allow a current of air to pass behind it. and so prevent the heat affecting the wall. When the voltage of the circuit is (say) 110, the resistance must be increased to absorb a further 10 volts.

It is not advisable to run two such lamps on less than a 100-volt circuit for the sake of avoiding the waste in resistance, - which, it will be noted, represents ten per cent of the total energy, - since the resistance has the beneficial effect of steadying the current passing through the lamps, and consequently the light given by them.

The light from arc-lamps has been described as white, and so much is this the case that colours can be matched thereby with the same readiness as by daylight, a fact which drapers and others arc not slow to avail themselves of.

The mechanism attached to each pair of carbons, it would at first appear, should be on clockwork principle, and arranged to iced the carbons together at a speed which will exactly meet the rate at which the points are consumed. In the early days of lighting such an arrangement was used, but it was soon found that the carbons, varying in quality throughout their length, required some means of feeding them together at varying speeds. This has been achieved electrically by taking advantage of the following principle. If the carbons (which, at the commencement, were one-eighth of an inch apart, and had passing across them 10 amperes requiring a voltage of 45) were not moved, they, on burning away, would shortly require more than 45 volts to maintain the 10 amperes, or, in time, to maintain any arc at all, so, on the difference of potential or voltage between the two points increasing to above 45, an electro-magnet as a by-pass automatically obtains sufficient current to start the mechanism and force the earbons together to the required distance.

The peculiar cup-shape of the positive carbon, called the "crater", throws the light directly in front of it, so that if the two carbons should be placed vertically, and the positive or cup-shaped carbon be uppermost, practically all the light will be thrown downwards. By reversing the current without affecting the position of the carbons, that is to say, by making the lower carbon positive, all the light is thrown upwards. The former, shown in Fig. 638, is the arrangement generally seen, with a globe of opalescent glass inclosing it. The latter, shown in Fig. 639, in which the carbons are merely shielded from the eyes of those below by a metal or thick glass cone, so that the source of light cannot be seen, but the rays therefrom proceed to the ceiling, whence they are reflected to the floor. Given, then, a good white ceiling, the light thrown down is as near an approach to daylight as has yet been found, since there is no brilliant spot, from whence it proceeds, to irritate the eyes.

lamp Resistance.

lamp Resistance.

Fig 638   Arc lamp with Half globe Reflector.

Fig 638 - Arc-lamp with Half globe Reflector.

Fig. 639   Inverted Arc lamp is called an inverted lamp

, in which the carbons are merely shielded from the eyes of those below by a metal or thick glass cone, so that the source of light cannot be seen, but the rays therefrom proceed to the ceiling, whence they are reflected to the floor. Given, then, a good white ceiling, the light thrown down is as near an approach to daylight as has yet been found, since there is no brilliant spot, from whence it proceeds, to irritate the eyes.

I stated just now that an arc is not formed in vacuo, as are incandescent filaments. But arcs have been used, and are likely to be more and more frequently used in a partial vacuum, whereby the consumption of carbon is decreased and the light rendered softer, although somewhat at the expense of purity of colour.