as the carbons waste away, the distance between their tips becomes greater, the voltaic arc is lengthened, 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 machine with continuous currents, like Gramme's machine, a new difficulty 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 peculiarity, 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 mechanism 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 magneto-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 succeeded in overcoming the difficulties 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-candles the wick is consumed in proportion to the consumption 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 sufficient 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 insulating substance, while separating the two carbons, would soon form an impassable barrier between the one and the other, and extinguish 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