Popular Science Monthly/Volume 11/August 1877/The Electric Candle
|THE ELECTRIC CANDLE.|
PUBLIC attention has been directed to Jabloshkoff's system of electrical lighting by the use that has been made of it at the Magasins du Louvre, in illuminating a hall recently opened. During the past year this invention was brought under the notice of the public by a communication addressed to the Paris Academy of Sciences, and by an experiment made before the Physical Society. The readers of La Nature are acquainted with the usual methods of producing electrical light, and we here again explain their general principles, with a view to render more intelligible the comparisons we propose to make.
Two carbon-points, borne on suitable metallic supports, are arranged in one line, with their tips in contact. An electric current of high intensity is made to pass into them; they may become heated, but they will not give out light unless they be separated by a little distance from each other. On separating them, by the hand or other-wise, the voltaic arc appears and gives out a very strong light. This light persists, provided the carbons are a few millimetres apart; but, 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 sufficient to melt and even to vaporize glass or kaolin, and thus the insulating septum between the carbons wears away simultaneously with them.
|Jabloshkoff's Electric Candle. (actual size).—C C, carbon-points of gas-coke; I I I I, insulating substance; T T, tubes holding the carbon-points; A, socket of asbestos holding the system together; F F, copper wires conveying the electric current.|
If the source of electricity gives constant currents, then, inasmuch as the carbons wear unequally, one wasting more rapidly than the other, the distance between the points will become too great, and the light will be extinguished. To overcome this difficulty, we have only to make the carbon that burns most rapidly twice as thick as the other.
It is true that hitherto the electric candle has worked better with magneto-electric machines giving alternating currents, than with piles or Gramme machines; in the former case the carbons wear away equally, and are of the same thickness.
To complete the description of Jabloshkoff's apparatus, we have to add that each carbon is socketed in a brass tube, connected with a wire coming from the source of electricity. These two tubes are attached to each other in various ways, according to the applications had in view; the one essential precaution is, to take care that they do not come into contact with each other.
The name candle has been very happily applied to this simple apparatus; it is, indeed, a candle with two wicks burning side by side, and which lower their luminous point as combustion goes on. One interesting peculiarity it possesses, namely, that the luminous point can be turned downward, so that there is nothing to throw a shadow. Its light may be modified by the use of opal or ground-glass shades.
How the Candle is lighted.—One of the principal advantages of Serrin's lamp is, that it can be lighted from a distance. The lamp is made ready, say, in the morning, and, when night comes, all that is required is to admit the electric current, and light is instantaneously produced. At first Jabloshkoff lighted his candle directly by supporting on the tips of the two wicks a piece of charcoal which he soon afterward removed; thus the voltaic arc was produced as in electric lamps, by beginning with contact, and then placing the two carbons at the required distance from each other. But it was necessary to devise some method for lighting the candle from a distance, and this fresh difficulty M. Jabloshkoff has overcome by a very simple device. He places between the two carbons a little bit of graphite, of the diameter used in lead-pencils; this acts as a conductor between the two wicks of the candle. On the current entering it, the bit of graphite soon becomes red-hot, and is burned up; there is then a break of continuity between the wicks, and the electric arc is produced. Instead of graphite, a fine metallic wire, or a bit of lead, can be used.
Relighting.—As we have stated, the insulating strip between the wicks is fused at the points near the voltaic arc, and so disappears gradually in proportion to the waste of the carbon-points. But this fusion of the insulator is attended with another consequence that but few of our readers would have anticipated. That which in its solid state is an insulator becomes in its liquid state a conductor, and allows of a longer span of the electric arc than could be had in the free air. Owing to this conductivity of the strip of kaolin, the circuit may be opened for a moment and the candle lighted again without any need of resorting to any of the contrivances already described under the head of "Lighting." But after a certain length of time, as the substance cools, it loses its conductivity, and then the candle cannot be relighted by simply closing the circuit again. We may extinguish the candle for nearly two seconds, and relight it by simply closing the circuit. Hence the electric candle may be used for transmitting telegraphic signals according to the Morse alphabet, by means of flashes of greater or less duration, divided by longer or shorter periods of eclipse. For such use the candle is better adapted than the electric lamp, as it is more readily relighted, producing at once a perfect voltaic arc, whereas in the lamp the arc is produced gradually.
Division of the Light.—Hitherto a separate pile, or a separate machine, has been necessary for the production of each electric light, and it has been found impossible to place two lamps in one circuit. This is readily understood when we consider the mechanism of the regulating apparatus. In electric lamps the approximation and the separating of tlie carbons are controlled by an electro-magnet, which itself follows the variations of resistance in the circuit produced by changes in the length of the voltaic arc. As the arc lengthens the resistance of the circuit is increased, and the electro-magnet is weakened, and allows the carbons to approximate. It is easily understood that if there are two lamps and two voltaic arcs in one circuit, and if only one of these arcs is lengthened, both electro-magnets will act and shorten the two voltaic arcs. The consequence is, that the second lamp will have its proper working interfered with, while the first alone should have been regulated. In other words, the solidarity of the two apparatuses will tend to produce in each unnecessary changes of regulation that will constantly result in causing the system of lamps to work badly. But with the candle there is nothing of this kind, and, provided that the source of electricity possesses sufficient tension to produce the voltaic arcs, many may stand in the same circuit. In the Magasins du Louvre we have seen in some instances four lights, in others three, produced by a single machine. The sequel will show whether we can reasonably expect to see even a greater division of the electric light, and whether this invention may not have still further applications.—La Nature.
- Translated from the French by J. Fitzgerald, A.M.