ELECTRIC LIGHT. 



ENGINEERING. 



279 



if. the object is to obtain a single light of great 

 brilliancy, machines of low resistance and large 

 quantity must be employed. If on the other 

 hand it is desired to obtain in the same circuit 

 several lights of moderate intensity, machines 

 of high internal resistance and of correspond- 

 ingly high electro-motive power must be used. 



Another mode of producing the electric light 

 is that devised by Messrs. Thompson and Hous- 

 ton, in which a succession of sparks is made 

 to yield a continuous light. A pair of carbon 

 pencils is mounted vertically, but, while the 

 positive carbon is fixed, the negative carbon 

 is capable of vibration. At first the two pen- 

 cils are in close contact, and the current passes, 

 of course, through them; but the movable rod 

 by its motion breaks contact, and a spark im- 

 mediately appears. Before the impression made 

 by this spark upon the retina has faded away, 

 the oscillating carbon springs back, whereby 

 contact is momentarily renewed, and as mo- 

 mentarily broken ; another spark then appears, 

 and, as these sparks succeed each other with 

 great rapidity, they give rise to a continuous 

 sensation of light. An electric light may thus 

 be obtained with a motor much too feeble to 

 produce the light in its ordinary form. 



Still another method, which, however, prom- 

 ises to be of little economic importance, is that 

 in which the electric discharge is sent through 

 a gas or vapor in a rarefied condition, when 

 luminous effects of great beauty are produced. 

 The phenomena are best seen in Geissler tubes 

 so named after an artist of Bonn who originally 

 devised them. These are hermetically sealed 

 glass tubes inclosing various gases in a highly 

 attenuated state, through which the sparks 

 from an induction coil can be passed by means 

 of platinum electrodes fused into the glass. On 

 the passage of the current a soft and delicately 

 tinted light streams through the tube from pole 

 to pole. Although the electric light in these 

 tubes is too feeble to be employed for ordi- 

 nary purposes of illumination, it has certain 

 special applications that give it some practical 

 value. Thus it has been used, to a limited ex- 

 tent, by medical men in examining the con- 

 dition of any cavity of the body into which it 

 is possible to introduce a properly shaped tube. 

 Its use has also been suggested in coal mines 

 where fiery seams are being worked ; and for 

 this purpose an ingenious lamp has been con- 

 structed by MM. Dumas and Benoit. It has 

 also been proposed to use Geissler tubes in 

 gunpowder factories, and as a means of sub- 

 marine illumination. 



The economy and suitability of the electric 

 light for illuminating large buildings and open 

 spaces may be considered as already amply 

 demonstrated, and many of the recently de- 

 vised systems, some of which we have briefly 

 described, promise to accomplish, more or less 

 successfully, that great object which has so 

 often proved a stumbling-block to the inventor 

 the divisibility of the light. It seems para- 

 doxical to say that the great disadvantage of 



the electric light lies in its excessive brilliancy. 

 Yet that is really the case. To temper its in- 

 tensity it is common to use shades of ground 

 glass ; but the production of an intense light 

 to be afterward deadened is obviously a waste- 

 ful process. It is not until the strong light can 

 be economically divided into several lights of 

 moderate intensity that it stands a chance of 

 becoming the domestic light of the future. 

 Many inventors are now at work on this prob- 

 lem, and it is the opinion of those most com- 

 petent to judge that success is not far distant. 



If an electric light of moderate power can 

 be cheaply obtained, its advantages over gas, as 

 at present burned, are beyond question. The 

 electric light, for instance, does not vitiate the 

 surrounding atmosphere as ordinary combus- 

 tion does. The carbon points burn away, it is 

 true, and thus consume oxygen and produce 

 carbonic-acid gas ; but the action is insignifi- 

 cant compared with that which takes place 

 during the production of the same amount of 

 light from candles, oil, or gas. Moreover, the 

 electric light can be produced in a closed vessel 

 from which air is excluded ; and thus the sur- 

 rounding atmosphere may be kept perfectly 

 free from contamination. Again, this light is 

 recommended by its exceptional purity. In a 

 gas-flame the yellow rays predominate, and 

 hence it becomes impossible by gaslight to dis- 

 tinguish, say, a bluish-green from a greenish- 

 blue. But by the electric light colors are much 

 more accurately discriminated. 



With the space allotted to this article, it has 

 been impossible to give either detailed accounts 

 of all the various forms of apparatus that have 

 been devised for producing the electric light, 

 or minute descriptions of the results that have 

 been obtained. Those wishing to go more 

 fully into the subject are referred to " Apple- 

 tons' Cyclopaedia of Applied Mechanics," to Mr. 

 George B. Prescott's recent work on "The 

 Speaking Telephone, Electric Light, and other 

 recent Electrical Inventions," and to M. Fon- 

 taine's treatise on " Electric Lighting." 



ENGINEERING. Several large works of 

 engineering are now approaching completion 

 or have been begun in the United States, which 

 country offers the best field at present for prog- 

 ress in this art. The opportunities have been 

 well utilized by the engineers of America, who, 

 while as a class they do not yield in knowledge 

 and theoretical schooling to their European 

 confreres, are much freer than they from tram- 

 meling professional traditions, and consequent- 

 ly bolder in their conceptions and more ingen- 

 ious in overcoming practical difficulties, Their 

 eminence has recently been acknowledged with 

 candor by several of the first English engineers 

 at their annual gathering, some of whom sided 

 with the Americans in the mooted question of 

 pin or rivet connections in iron truss bridges 

 of long span. The narrow gauge on railways, 

 which the United States first adopted, is being 

 rapidly introduced abroad, where the condi- 

 tions of the land or of population make it 



