When it becomes a question of practical lighting, it is very certain that the best electric lamp will be the one that is most simple and requires the fewest mechanical parts. It is to such simplicity that is due all the success of the Jablochkoff candle and the Reynier-Werdermann lamp. Yet, in the former of these lamps, it is to be regretted that the somewhat great and variable resistance opposed to the current in its passage through two carbons that keep diminishing in length, in measure as they burn, proves a cause of loss of light and of variation in it. And it is also to be regretted that the duration of combustion of the carbons is not longer; and, finally, it is allowable to believe that the power employed in volatilizing the insulator placed between the carbons is prejudicial to the economical use of this system. In order to obviate this latter inconvenience, an endeavor has been made in the Wilde candle to do away with the insulator, but the results obtained have scarcely been encouraging.
An endeavor has also been made to render the duration of the carbons greater by employing quite long ones, and causing these to move forward successively through the intermedium of a species of rollers, or of counterpoises, as in the lamps of Mersanne and Werdermann; but then the system becomes more complicated. Finally, in order to keep the resistance of the carbons at a minimum and constant, their contact with the rheophores of the circuit has been established at a short distance from the arc, and this is one of the principal advantages possessed by the Reynier-Werdermann system. At a certain epoch it was thought that the problem might be simply solved by arranging in front of each other two carbons actuated by a spiral spring, as in car lamps, and kept at a proper distance apart for forming the electric arc by two funnel-shaped pieces of calcined magnesia, into which they entered like a wedge in measure as their conical point were away through combustion. This was the system of Mr. De Baillehache, and the trials that were made therewith were very satisfactory. But, unfortunately, the magnesia was not able to resist very long the temperature to which it was submitted.
The problem found a better solution in the sun-lamp but has been solved in another manner, and just as simply, by Mr. Solignac, and the results obtained by him have been very satisfactory as regarded from the standpoint of steadiness of the luminous point.
In this system, a general view of which is given in Fig. 1, and the arrangement in Figs. 2 and 3, the carbons, F F, which are horizontal and about fifty centimeters in length, are thrust toward each other by two barrels, K, K, which wind up two chains, E, E, passing around the pulleys, D, D, fitted to the extremities of the carbons. These latter are provided beneath with small glass rods, G, G, whose extremities toward the arc abut at a short distance from the latter against a nickel stop, L (Fig. 3), which supports them, moreover, at M, by means of a tappet whose position is regulated by a screw. The current is transmitted to the carbons by two friction rollers, I, I, which serve at the same time as a guide for them, and which give the electric flux a passage of only one or two centimeters over the front of the carbon to form the arc. Finally, the whole is held by a support, A, and two pieces, CB, CB, which at the same time lead the current to the friction rollers through projections, J. The two systems are made to approach or recede from each other, in order to form the arc, by means of a regulating screw, H.
At present, the lighting of these lamps is effected by means of this screw, H, but Mr. Solignac is now constructing a model in which the lighting will be performed automatically by means of a solenoid that will react upon a carbon lighter, as in several already well known systems.
If the preceding description has been well-understood, it will be seen that the carbons are arrested in their movement toward each other only by the glass rods, G, abutting against L; but, as the stops, L, are not far from the arc, and as the heat to which they are exposed is so much the greater in proportion as the incandescent part of the carbons is nearer them, it results that for a certain elongation of the arc the temperature becomes sufficient to soften the glass of the rods, G, G, so that they bend as shown at O (Fig. 3), and allow the carbons to move onward until the heat has sufficiently diminished to prevent any further softening of the glass. In measure as the wearing away progresses, the preceding effects are reproduced; and, as these are produced in an imperceptible and continuous manner, there is perceived no jumping nor inconstancy in the light of the arc. Under such conditions, then, the regulation of the arc is effected under the very influence of the effect produced; and not under that of an action of a different nature (electro-magnetism), as happens in other regulators. It is certain that this idea is new and original, and the results that we have witnessed from it have been very satisfactory.
There is but one regulation to perform, and that at the beginning, but this once done the apparatus operates with certainty, and for a long time. With a Meritens machine of the first model it has been found possible to light five lamps of this kind placed in the same circuit.
According to the inventor, this lamp will give a light of 100 carcels per one horse-power, and with a three horse-power six lamps may be lighted; but we have made no experiments to ascertain the correctness of these figures.
As for the cost of the glass rods, that amounts to one franc per two hundred meters length. They can, then, be considered only as an insignificant expense in the cost of the carbons. We consequently believe that it will be possible to employ this system advantageously in practice.--Th. du Moncel.