CHAMBERS'S INFORMATION FOR THE PEOPLE. 



consists of two, three, or more concentric Argand 

 burners, each inner one rising a little above the 

 outer. The Bude Light depends upon introducing 

 oxygen into the centre of the flame, instead of air, 

 as in the common Argand. 



The Lime-ball Light, sometimes called the 

 Drummond Light, is produced by directing the 

 flame of a mixture of oxygen and hydrogen 

 against a piece of lime. The flame is itself pale, 

 but the intense heat it communicates to the lime 

 makes the latter give out a light rivalling that of 

 the sun. When gas is passed through naphtha, it 

 becomes saturated with naphtha vapour, which, 

 being exceedingly rich in carbon, greatly increases 

 the illuminating power of the gas. 



ELECTRIC LIGHTING. 



So great progress has recently been made in 

 the methods for producing and regulating the 

 electric light, that keen public interest in the 

 question of its availableness for general purposes 

 has been awakened; For the fundamental truths 

 of electricity and the cognate subjects, the reader 

 is referred to Vol. I. of this work, at page 257 and 

 following pages. Every circuit offers resistance 

 to an electric current flowing through it, and is 

 in consequence heated. If a small portion of the 

 circuit is made of very high resistance, the heat 

 generated in that portion may be sufficient to 

 make the material there so hot as to become 

 luminous. This is the essence of all electric 

 lighting. The part of high resistance may be a 

 thin strip of a solid conductor, as in Swan's, 

 Edison's, and Maxim's so-called incandescent 

 lights ; or it may be gaseous, as in the familiar 

 Geissler vacuum tubes and the ordinary arc 

 electric lights, of which Siemens', JablochkofFs, 

 Brush's, Serrin's, and Jamin's may be cited as 

 examples. For practically useful lighting, a strong 

 current is a necessity. Originally this was ob- 

 tained from a large number of voltaic cells ; but 

 these were found so troublesome and costly to 

 keep in good condition, that the electric light 

 remained long a scientific curiosity for lecture- 

 room or laboratory. Faraday's discoveries, how- 

 ever, indicated a new method of producing 

 currents ; and the great recent improvement in 

 the construction of magneto-electric machines 

 has quite changed the aspect of the problem of 

 electric lighting. All these machines depend 

 upon the induction of currents in coils moving 

 in the vicinity of magnets, or more generally 

 in any magnetic field however produced. The 

 direction of the induced current depends upon 

 whether the coils are moving from a weaker to 

 a stronger or from a stronger to a weaker region 

 of the magnetic field. In the earlier forms of 

 magneto-electric machines, the coils were made 

 to rotate rapidly in front of permanent steel 

 magnets. More widely employed now are the 

 more recent dynamo-electric machines, or simply 

 dynamos, in which electro-magnets are substi- 

 tuted for the permanent steel magnets. The soft 

 iron cores of the electro-magnets never wholly 

 lose their magnetism. Hence the rotation of the 

 coils in their vicinity gives rise at first to a very 

 feeble current, which is directed through both the 

 external circuit and the electro-magnet coils, 

 thereby increasing the magnetisation of the cores, 

 and so intensifying the original induced current. 



496 



The current and the magnetisation thus act and 

 react upon one another, growing steadily until 

 they have each reached a maximum, which 

 depends on the rate of rotation of the coils and 

 the resistance of the whole circuit. The current 

 so produced may be continuous and in one 

 direction, or discontinuous and in rapidly alternat- 

 ing directions, according to the manner in which 

 the contacts are adjusted at the terminals of the 

 coils. The rotation of the coils is usually 

 effected by a steam or gas engine, so as to obtain 

 a rapidity of motion sufficient to produce a strong 

 enough current. 



In electric lights of the arc type the current 

 passes through the air between two pieces of 

 carbon which are attached by wires to the ter- 

 minals of a Gramme, Siemens, Biirgin, Brush, or 

 other machine. In order to allow the current to 

 work up to its maximum strength, the carbons 

 must first be brought into contact, and then with- 

 drawn to the required distance. If the distance 

 is too great, the resistance in the circuit is made 

 too high, the current ceases to flow, and the light 

 is extinguished. Hence every practically useful 

 light is supplied with an automatic regulator, 

 which keeps the carbons apart as long as the 

 current is flowing, and brings them into contact 

 again if through any accident the current is 

 broken. By its means also the carbons are kept 

 at the necessary distance apart, in spite of their 

 inevitable dissipation under action of the intense 

 heat to which they are subjected. The carbon 

 from which the current passes wastes away 

 much faster than that to which the current sets. 

 Hence, in Jablochkoff's electric candle, which 

 has the carbons side by side, separated by a strip 

 of a non-conducting material, an alternating 

 current is used, so that both carbons waste away 

 equally. In Werdermann's light there is no 

 appreciable waste on the upper carbon, which is 

 a comparatively broad plate, against which the 

 lower thin pencil-shaped carbon presses. This 

 form of light is one in which the subdivision of 

 the electric light, long an insuperable difficulty, 

 has been very successfully accomplished. For 

 ordinary domestic purposes, however, the arc 

 light is in general too brilliant to be of any real 

 use, and is limited to the lighting of streets, 

 squares, railway stations, large halls, and light- 

 houses. More suitable for ordinary house illum- 

 ination are the incandescent lamps of Swan, 

 Edison, and Maxim, which differ only in the 

 mode of preparation. They all consist of 

 specially prepared thin carbon filaments, which 

 become incandescent under action of the travers- 

 ing current. To reduce their dissipation to a mini- 

 mum, they are inclosed in hermetically sealed 

 vacuum globes, through the bottom or top of 

 which the platinum terminals are led to the out- 

 side. Such lamps are worked with alternating 

 currents, otherwise the carbon filament would 

 tend to thin out at the positive end, and ultimately 

 break. Theatres, lecture halls, and large steamers 

 have already been most successfully lit up with 

 Swan's lamps, which are unsurpassed for their 

 beautiful and peculiarly mild light. 



Faure's accumulator, an improved form of 

 Planters secondary battery, has recently been 

 applied as the source of energy in electric 

 lighting, and may in certain circumstances super- 

 sede the magneto-electric machine. 



