42G 



* KNOWLEDGE 



[March 17, 18fc: 



stance, Ruch as finn platinum wire, then, on passing the 

 current, heut is producinl in the thin conductor, because of 

 the o]>positioii or r'ulnlmicr its thinness j)rcsctits to thf 

 current Tiiis liciit ncciimuliiti's, so to spcuk, with the 

 current, and, idtiiiiat^'ly, the tliin conductor (?iiiits n pure 

 whit«' lif;ht. Our renders must please understand that 

 liglit is l>ut the manifestation of a considerable exaltation 

 of t»'m]i('raturp. 



We have mentioned platinum as the thin conductor ; the 

 reason for this is that nil metals, A-c, even when of the 

 same size, do not conduct with the same readiness or 

 facility, copper V)einf; about the best, and platinum the 

 poorest of metals. It then follows that a thin platinum 

 wire (resembling the small pifie with sand in it) will offer 

 more resistance than a copper wire of the same gauge, and, 

 accordingly, will get hotter, and so give out more light. 

 Even, however, if the copper were equal in its resistance, 

 its physical features would, in this instance, preclude its 

 use. It would, in fact, melt, or even volatilise. 



Such was the lirst form of incandescent lighting. It 

 did not, however, prove very satisfactory. Bodies offer- 

 ing more resistance than jilatinum were required, and it 

 is only natural that carbon in some form or another should 

 be re.sorted t<i. It was found, on trying it, that union 

 was made with the oxygen of the air, converting the fila- 

 ment into carbonic anhydride gas. It occurred to Mr. 

 Swan about twenty years since that, if a jierfect vacuum 

 could be obtained, the carbon would be made more durable, 

 as there would be no oxygen for it to combine with. It is 

 on this ground that Mr. Swan claims, and is almost 

 universally considered to be, the prime inventor of the in- 

 candescent lamp. The necessary vacuum, however, could 

 not then be obtained, but jnore recent inventions 

 in pneumatics have enabled electricians to achieve their 



Fig. 1. 



object Even with tliis vacuum, it was found that the 

 filaments were not so durable as had been hoped, and Mr. 

 Swan must again be credited with discovering the reason. 

 He demonstrated the cause to be the presence of particles 

 of air, itc, located in the spaces separating the particles 

 of the filaments, and by raising his filament to a very high 

 temperature he expelled the air. In consequence, we 

 secure, to all intents and purposes, a perfect vacuum. 

 There being nothing with which the carbon may unite, 

 th<: supposition forced upon us is that the filament 

 should last for ever. Such an idea, however, is 

 erroneous. Doubtless our readers are aware that in an 

 arc lamp the two carbon rods are consumed, one of them 



twice n.s rapidly as the other. This phenomenon ia to 



some ext<'nt due to the current, in passing, t^-aring away the 

 particles of carbon from one rod, and depositing them 

 on the other. This also occurs in incandescent lamps, 

 although to a modified and very much smaller extent The 

 current, in pa.ssing through the filament, tears away minute 

 particles on entering and deposits tlurm on leaving, so that 

 one end is gradually diminished and eventually fracture<l. 

 This, however, is a process of time ranging from COO to 

 1,000 hours continuous burning. 



The different forms of lamp exhibited at Sydenham are! 

 the Swan, Edison, Lane-Fox, Maxim, and British. 



The Swan (Fig. 1) is the invention of ilr. Swan, of New- 

 castle-on-Tyne. It consists of a glass globe nearly 2 in. 

 in diameter. A glass rod, C D, with the two platinum 

 wires, A and B, passing through it, is fused into the globe. 

 The inside ends of the wires are attached to the ends of the 

 carbon filament, which is made from cardboard (Bristol 

 board), and the free or external ends are connected to 

 wires from the machine generating the current. 



Fig. 2 represents an " Edison " lamp. It consists of a 

 glass bulb, about 5 in. Jong. The filament is made of 

 lianiboo tibre, one end being, by a very ingenious device, 

 made slightly thicker than the other. Platinum wires are 

 attached to the filament, the junctions being coated, elec- 

 trically, with a little copper. The platinum wires are 

 fused into the glass tube AB. at B, and are shown in the 

 diagram to be continued, through a kind of cap of plaster 

 of Paris, to a brass band, T, a)id brass screw-thread, T, 

 respectively. The lamp is then screwed into a correspond- 

 ing socket, so that T is in contact with another screw- 

 thread, having attached to it a wire from tlie machine. 

 The other machine wire is connected to a brass band in 

 the socket which comes into contact with T, and se 



