430 THE POPULAR SCIENCE MONTHLY. 



as the carbons waste away, the distance between their tips becomes 

 greater, the voltaic arc is lengtliened, and soon the light goes out, 

 unless the points are again brought near to each other. Hence it is 

 seen that this rudimentary apparatus cannot support the electric light 

 for over a few minutes, and some contrivance had to be devised for 

 approximating the carbons in proportion as they waste away, and for 

 keeping them a very small distance apart. This is done in the lamps 

 devised by Serrin, Foucault, and others. 



When the source of electricity is a pile or a magneto-electric ma- 

 chine with continuous currents, like Gramme's machine, a new dilficulty 

 is met with ; for here the two carbons are consumed unequally, the 

 positive one wasting about twice as fast as the negative. On the other 

 hand, machines with alternately reversed currents present this peculi- 

 arity, that in them the waste of the two carbons is equal. 



To whatever grade of perfection such lamps may have attained, 

 they undoubtedly labor under sundry disadvantages. Their mechan- 

 ism is delicate, and necessitates very great care on the part of those 

 who operate them. It is not very easy to regulate them. Their main 

 bulk, being situated beneath the luminous point, casts an objectionable 

 shadow. As usually constructed, their size is such that they cannot 

 work over three hours without having fresh carbons put in, and this 

 renewal of the carbons necessitates either a temporary interruption of 

 the lighting or else the keeping of an extra machine, which involves 

 an increased outlay of money. Finally, the price of such machines is 

 pretty high, and can hardly be reduced. 



The very great progress made during the last few years in the 

 construction of masrneto-electric machines has made more evident the 

 imperfections of the regulating apparatus. 



Such was the condition of things when a Russian engineer, M. 

 Jabloshkoff, succeeded in dispensing altogether with the mechanism 

 of electrical lamps. Let us see how this lucky inventor has suc- 

 ceeded in overcoming the diiRculties that successively arose before 

 him. 



First of all, he sets out with the idea that the carbons must be 

 placed side by side, so as to consume them simultaneously without 

 having continually to regulate their respective positions, just as in 

 stearine-candlcs the wick is consumed in proportion to the consump- 

 tion of stearine. The first requisite is, that the voltaic arc shall be 

 produced only at the tips of the carbons. For this purpose it is suf- 

 ficient to place between the two carbons a strip of glass, kaolin, or 

 any other insulating substance, somewhat wider than the carbons, 

 and not reaching to their tips. It might be supposed that this in- 

 sulating substance, while separating the two carbons, would soon 

 form an impassable barrier between the one and the other, and ex- 

 tinguish the voltaic arc by requiring it to make too great a span. 

 But such is not the case ; the high temperature of the voltaic arc is 



