Every-day Science: Volume 6. The Conquest of Nature

by Henry Smith Williams; Edward Huntington Williams

In Sir Humphry Davy's experiments with his arc light he was led to believe that the light between the two points of carbon would be produced even in an absolute vacuum, if it were possible to create one. Several scientists at the time disputed this contention, and M. Masson, Professor of Physics in the École Centrale des Arts et Manufactures in Paris was particularly active in combatting the idea, maintaining that the arc had the same cause as the electric spark—the transport by electricity of the incandescent particles of the electrodes through the atmosphere. It was certain, at any rate, that no light was produced when the opposing carbons were brought into contact with each other, or were, on the other hand, separated too widely; and since there was a constant wearing away and shortening of the points, and thus a constantly increasing space between them, the great difficulty in making a practical lamp lay in regulating this distance automatically. It was finally accomplished, however, by the invention of a Russian officer, M. Jablochkoff, in 1876. The "Jablochkoff candle," as his lamp was called, marked an epoch in the history of electric lighting. One great merit of this invention was its simplicity, and while it has long since gone out of use, having been superseded by still simpler and better devices, it must always be recalled as an important stepping-stone in the progress of artificial illumination.

The name "candle" for Jablochkoff's lamp was suggested by the fact that the two carbons were placed side by side, instead of point to point, the light at the top thus suggesting a candle. Between these two carbons, and extending their whole length except at the very tips, was an insulating material that the arc could not pierce, but which burned away at a rate commensurate with the shortening of the carbons. In this manner the points were kept constantly at the proper distance without regulating-machinery of any kind. This ingenious apparatus had the additional advantage that it could be placed on any kind of a bracket or chandelier that was properly wired, thus dispensing with the cumbersome frames and machines of the point-to-point carbon arc lights then being introduced.

One difficulty at first encountered in using the Jablochkoff candle was the starting of the voltaic arc. In doing this it was necessary that contact be made between two carbon points, whether they lie parallel or point to point, and the necessary slight separation for producing the light effected later. To accomplish this Jablochkoff joined the tips of the carbons of his candle with a thin strip of carbon, which quickly burned away when the current was turned on, leaving the necessary space between the points for the arc.

There was one difficulty with the "candle" that seemed insurmountable for a time—the wasting of the two carbons was unequal, as in any arc light, the points thus gradually drawing apart until the passage of the current was no longer possible. To overcome this the rapidly wasting positive carbon was made double the thickness of its mate; but while this answered fairly well the thinner negative carbon gradually became heated by the increased resistance, and burned up too rapidly. The difficulty was finally overcome by the simple expedient of alternating the flow of the current, so that each carbon was alternately a positive and a negative pole. As the magneto-electric machines then in use produced alternating currents it was only necessary to use such machines for generating the current to produce an equal destruction of both carbons.

The simplicity and excellence of the light of these "candles" brought them at once into general popularity, not only in the large cities of Europe, but in many out-of-the-way places. Greece, Portugal, and other obscure European countries adopted them, and even Brazil, La Plata, and Mexico installed many plants. But stranger still, they were soon used for illuminating the palaces of the Shah of Persia and the King of Cambodia, and a little later were introduced into the residence of the savage King of Burma. In short, their use became universal almost immediately.

THE IMPROVED ARC LIGHT

About the time that Jablochkoff's candles were making such a sensation in Europe, Charles F. Brush, of Cleveland, Ohio, invented an arc light in which the carbons were set point to point, the distance being maintained and the necessary feed produced automatically in much the same manner as in the lamps used at present. Other inventions soon followed, some of the lamps being regulated by clockwork, some by electricity and magnetism.

The advantage of this type of arc lamp over the candle type—an advantage that led to its general adoption—was largely that of efficiency, a far greater amount of light being obtainable from the same expenditure of power by the point-to-point type of lamp.

In this lamp it is necessary that the points of carbon shall come in contact when the current is off, but be drawn apart a moment after the current is turned on, and remain at this fixed distance. To accomplish this, the lower carbon is usually made stationary, the feeding being regulated by the position of the upper carbon. In the usual type of modern lamp the passage of the current causes the points to separate the required distance through the action of an electromagnet the coils of which are traversed by the current. A clutch holds the carbon in place, the position of this being also determined by an electromagnet. The action is regulated by the difference in the resistance to the passage of the current caused by the increase in the separation of the points.

In the older type of arc lamp it was necessary to "trim" the lights by replacing the carbons every day; but recently lamps have been perfected in which the carbons last from one hundred to one hundred and twenty hours. In these the arc is enclosed in a glass globe which is made as nearly air-tight as possible with the necessary feed devices. This closed chamber is fitted with a valve opening outward, which allows the air to be forced out by the heat of the lamp, but does not admit a return current. In this manner a rarefied chamber is produced in which the carbons are oxidized very slowly; yet there is no diminution in the brilliancy of the light.