ELECTRIC LIGHT. 



269 



sity sent through it, the platinum links will 

 glow vividly, while the links of silver will re- 

 main dark and comparatively cool. From its 

 low electric conductivity, or, what amounts to 

 the same thing, from its high resistance, plati- 

 num is peculiarly fitted for exhibiting incan- 

 descence. 



Another condition influencing the amount 

 of resistance is the size or thickness of the 

 conductor. Eeduce the thickness, and the re- 

 sistance is immediately increased, with a cor- 

 responding increase in the production of heat, 

 the latter being also closely connected with 

 the strength of the current. Hence a pow- 

 erful current sent through a small platinum 

 wire immediately renders it incandescent, and 

 many attempts have been made to utilize 

 this metal for the production of the electric 

 light. Thus far, however, these have been 

 attended with little success, for the reason that 

 if the current driven through the thin wire 

 or strip of platinum is sufficiently powerful 

 the heat rises to such an intensity that the 

 metal is fused, melting down like a mass of 

 wax in the flame of a candle. When this 

 occurs the electric circuit is of course imme- 

 diately broken, and the current consequently 

 interrupted. To avoid such a result some con- 

 ducting substance capable of becoming incan- 

 descent, but that would not fuse at the high- 

 est heat, was required. Carbon answers these 

 conditions, and has accordingly been used in 

 one form or another for the production of the 

 electric light ever since the famous experiments 

 of Davy, made nearly seventy years ago. In 

 this mode of producing the electric light the 

 continuity of the circuit remains unbroken, 

 the light being obtained by connecting the 

 conducting wires with some substance like 

 platinum or carbon, so shaped as to offer in- 

 creased resistance to the passage of the cur- 

 rent, and by which it is thrown into a state of 

 incandescence. 



But the various means of obtaining light 

 by the incandescence of a resisting medium 

 be that medium a metal of low conducting 

 power like platinum, or a thin rod of carbon 

 are materially different from those employed 

 in producing what is specially known as the 

 electric light. This light was first obtained by 

 Sir Humphry Davy, in 1813, while experi- 

 menting with the great battery of the Royal 

 Institution in London. This battery consisted 

 of 2,000 zinc and copper couples, exposing an 

 aggregate surface of 128,000 square inches. 

 Davy found that when the current from this 

 pile was passed between two pointed pieces of 

 wood-charcoal attached to conducting wires, a 

 light next in brilliancy to sunlight was pro- 

 duced. He also found that this intense light 

 was accompanied by intense heat. Many sub- 

 stances which had previously been regarded as 

 infusible were melted and even volatilized when 

 placed in the luminous focus. Like effects 

 were produced in a vessel exhausted of air or 

 in an atmosphere of nitrogen or carbonic-acid 



gas. Hence it was concluded that the light 

 did not proceed from ordinary combustion. 

 In this as in the former case the light and heat 

 are the result of the resistance met with by 

 the electric current in its passage through the 

 circuit, the difference being that here the re- 

 sistance is due to a gap in the circuit which 

 the current is forced to bridge. The interval 

 between the charcoal points offers a great ob- 

 stacle to the passage of the current, and it is 

 to the gathering up of the force necessary to 

 burst across this interval that the intense heat 

 and light are due. The smallest space of air 

 is usually sufficient to stop the current, but 

 when the carbon points are first brought to- 

 gether and then separated it is seen that glow- 

 ing particles of carbon are constantly spring- 

 ing across the gap from pole to pole. Although 

 this takes place in both directions, the prevail- 

 ing course is from the positive to the negative 

 electrode. In fact, the positive carbon becomes 

 much more highly heated and wears away 

 much more rapidly than does the negative car- 

 bon. These solid particles of incandescent 

 carbon are believed to form a sort of conduct- 

 ing chain between the poles, by the aid of 

 which the current passes. It will therefore be 

 seen that this mode of illumination is, after all, 

 like the other, largely due to incandescence. 



In order to produce this form of electric 

 light the carbon poles are first brought to- 

 gether and then separated to a little distance, 

 the space across which the light can leap de- 

 pending on the power of the current. With 

 the battery before alluded to Davy was able to 

 obtain a current that in the open air would 

 leap a space of four inches between the car- 

 bon points, and this was lengthened to seven 

 inches in a vacuum. The luminous portion of 

 the circuit which forms a bridge between the 

 two solid poles is usually a little bent, and hence 

 receives the name of the voltaic arc. The color 

 and shape of this luminous arc depend on the 

 nature of the poles, but the light comes chiefly 

 from the glowing carbon points, while a pale- 

 blue flame fills the intermediate space. 



As we have seen, the form of carbon used by 

 Davy in his original experiments was wood- 

 charcoal, but this wasted away so rapidly that 

 it was of no practical use outside the laborato- 

 ry and the lecture-room. In fact, the electric 

 light amounted to little more than a brilliant 

 experiment until thirty years later, when Fou- 

 cault proposed to substitute for charcoal the 

 variety of carbon which is deposited in the in- 

 terior of gas-retorts and is known as gas-car- 

 bon. This is produced by the decomposition 

 of dense gaseous hydrocarbons, at the high 

 temperature at which the distillation of coal 

 is carried on. The gas-carbon has a laminated 

 structure, is very hard and solid, and when 

 sawed into rods or pencils is used with advan- 

 tage in producing the electric light, since it is 

 vastly more durable than the softer forms of 

 carbon, such as wood-charcoal. But gas-carbon 

 also has its disadvantages, being rarely pure or 



