VOLTAIC ELEGTUI' 



177 



VOLTAIC ELECTRICITY. XI V. 



TH K.ICTBIC LIGHT TH ABC 8Y8TKM THE JABLOCHKOFF 

 CANDLK THE INCANDESCENT 8Y8TC1C DOMESTIC ILLU- 

 MINATION BY ELECTCl' 11 >. 



HAVING already traced the history and gradual development 

 of the dynamo-electric machine, wo shall now bo in a position 

 to consider the general question of electric illumination, which 

 is jnat now of so ranch interest. The newspapers have for the 

 last few yean so teemed with advertisements of electric- 

 lighting companies, and with accounts of new methods of 

 electric illumination, that the ordinary reader is altogether 

 puzzled to know how so many different plans of arriving at the 

 same end can bo possible. But without any difficulty it is 

 possible to sort all the forms of electric lamps now before 

 the public into two groat divisions namely, arc lights and 

 incandescent lights. In outward appearance these two forms 

 of light differ very considerably. The arc light with its blue- 

 white painful glare is generally used for our streets, or to 

 illuminate very large areas, such as the reading-room at the 

 British Museum, which is lit by four largo hanging lamps 

 of that nature. The incandescent lamp, on the other hand, 

 gives a soft, steady, yellow radiance of from five candle to 

 twenty-five candle power, which is well adapted for the 

 illumination of private houses. Lot us at once endeavour 

 to make clear the method by which such different results are 

 achieved. 



The first arc light was produced quite at the beginning 

 of this century by Sir Humphry Davy. Noticing that the 

 wires from a powerful electric battery gave sparks, and that the 

 two extremities of the wires became hot when brought together, 

 he tried the experiment of furnishing each wire, or electrode, 

 with a short pencil of charcoal. The result was that the 

 charcoal points when brought together became red hot, and 

 if separated from one another by a short distance, a brilliant 



Fig. 83. 



light played between them, as shown in Fig. 83. The immense 

 battery used by Davy allowed this distance to amount to four 

 inches, and the luminous atmosphere between them, owing 

 probably to the heated air, took the form of an arch, hence the 

 term arc, or arc-light. Exactly the same effect is of course 

 produced if instead of a battery current that from a dynamo 

 machine is employed. 



Foucanlt substituted gas retort carbon, or coke sawn into 

 pencils, for the charcoal which Davy employed. This material 

 is so hard that it wastes away slowly, and has been used for 

 electric lamps ever since. But of late years a more homogeneous 

 carbon has been produced by manufacture, and these are now 

 almost exclusively employed. The arc-light requires a regulator, 

 which acts the same part as a candlestick does to a candle. 

 But there are certain points to be observed which require this 

 regulator to fulfil particular conditions. In the first place, 

 the arc is not produced at all unices the carbon points first 

 touch and are then separated, and if from any cause the light 

 should be extinguished, the pencils must be again brought into 

 contact before the arc can be re-established. The general form 

 of arc-lamp is shown at Fig. 84. The two carbon rods are 

 placed vertically one above the other, and held in sockets. 

 Their points touch until the connection is made with the source 

 of electricity. Directly this occurs, and the current flows from 

 one pencil to the other, they are seen to grow white hot, and 

 by the action of an electro-magnet they are separated, and the 

 arc is established between them. As they gradually waste 

 142 N.E. 



Fig. 84. 



away they are caused to approach one another by suitable 

 mechanism until they are consumed. Regulators have been 

 devised by Foucault, Dobosoq, Benin and, more lately, by 

 Siemens, Bruith, and others -all conttiuing nelf-adjosting 

 mechanism to fulfil the conditions 

 stated. 



Although, as wo have seen, the 

 carbon rods of the arc form of 

 electric light are generally placed 

 vertically one above the other, OH 

 shown in Fig. 84, many other 

 positions for them have been 

 adopted by different inventors ; 

 indeed, it would be somewhat 

 difficult to devise a new one for 

 them. In Werderman's electric 

 regulator one carbon takes a ban- 

 like shape, the other being a 

 very thin carbon rod which, by 

 means of a counterweight, is kept 

 constantly pressed against it. In 

 the Wallace- Farmer regulator, the 

 carbons are in the form of two 

 plates, about seven inches long 

 and three broad. The distance 

 between these plates is regulated 

 by an electro-magnet, and the arc 

 plays between them. In another form of light recently in- 

 troduced, the points of the carbon rods are inserted in holes in 

 a marble block, which block becomes white hot when the 

 current is applied. This form of arc-lamp is perfectly steady, 

 which is a groat deal more than can be said of many others. In 

 the Rapieff electric lamp four carbon rods are employed, which 

 converge to one point where the arc is established. The ad- 

 vantage claimed for this arrangement is that a spent rod can 

 be replaced by a fresh one without extinguishing the light. 

 The Eapieff form of lamp has been in use for some time in 

 the composing rooms of the Times newspaper, where it usefully 

 supplies artificial daylight for work which must obviously be 

 done during the dark hours. 



Many of the older forms of regulators were so intricate 

 in construction, and therefore so expensive, that attempts 

 were made to produce an arc-light in a more simple manner. 

 This was achieved a few years ago by the Russian engineer 

 Jablochkoff, and the introduction of the so-called " Electric- 

 Candle " invented by him had more to do with the revival of 

 electric illumination as a near possibility than anything else. 

 not excepting even the Gramme machine. The Jablochkoff 

 candle is entirely free from mechanism, for the provision for 

 the maintenance of the carbons in their respective position-* 

 is contained in the carbons themselves. These, instead of 

 being one above the other as in the old lamps, are placed side 

 by side, but separated by a thin layer of plaster of Paris. 

 Each "candle" is bridged over at the top by a conducting 

 link of graphite, so that when the current is applied to the 

 base of the twin carbon rods, this graphite is quickly fused, 

 and the arc is established between the carbons. As they waste 

 away, the plaster partition is fused with them, and the com- 

 pound rod burns down just as a candle would. 



Under ordinary circumstances the consumption of the 

 positive carbon is in a given time double that of the negative 

 carbon, and in the older forms of regulator this was provided 

 for by the one rod being fed towards the arc at double the rate 

 of the other. But this only holds good where a battery current 

 or that from a direct current machine is employed. It is 

 obvious that if a Jablochkoff candle were dependent upon such 

 a machine or battery it would soon be extinguished, for one 

 bide would waste at double the rate of the other. This has 

 been obviated by making the positive rod twice the substance 

 of the other, but the general plan is to employ an alternate 

 current machine. When the Jablochkoff candle was first 

 publicly shown in this country, an old-fashioned alliance 

 machine was put to work for the purpose, bu- latterly Gramme 

 and others have produced modified machines which give alternate 

 currents. Such machines are now in use on the Thames 

 Embankment, and in other situations where the Jablochkoff 

 candle is regularly used. A formidable objection to this 

 system of lighting is the circumstance that if one candle from 



