Littell's Living Age/Volume 135/Issue 1740/Miscellany
Influence of Light on the Electric Resistance of Metals. — Some two or three years ago we were startled by the announcement that the electric conductivity of selenium is capable of being affected by light. It has, however, been since determined that not only selenium but also the allied element tellurium has its electric resistance diminished after exposure to luminous rays. Desirous of determining whether other bodies are similarly affected, Dr. Börnstein has carried out some interesting researches in the Physical Institute at Heidelberg. His results are published in a paper which has been translated by Mr. R. E. Day, in the Philosophical Magazine (June, 1877, p. 481). Gold, silver, and platinum are the only metals which Dr. Börnstein has yet examined, but as he finds that all these are sensitive to light, he is inclined to infer that the property, so far from being exceptional, is one enjoyed in greater or less measure by all metals. In the case of selenium and tellurium, it has been suggested that the alteration of resistance is due to the action of calorific rather than of luminous rays. But no such objection can be urged against the experiments with the noble metals. In fact, the resistance of these metals increases with the temperature, so that when it is found that on exposure to direct light the resistance is diminished, it is clear that heat can have-nothing to do with causing such a change. Heat, indeed, tends to mask the effects of light, and the diminution of resistance is therefore a differential effect; an effect representing the difference between the increase of resistance consequent on rise of temperature, and the decrease of resistance due to the action of light. Dr. Börnstein’s experiments, therefore, show beyond question that the electric conductivity of the noble metals is exalted, or their resistance diminished, by the direct effect of luminous rays. Popular Science Review.
The Electric Candle — Experiments have been recently conducted at the West India Docks with the view of testing the illuminating power of the so-called electric candle devised by M. Paul Jablochkoff. This simple means of producing a steady electric light consists in placing two carbon pencils side by side, but separated by a bar of a composition called "kaolin." On the passage of the current the carbons slowly burn down, and the kaolin is consumed by the heat at exactly the same rate. The carbons are thus kept always at the same distance apart, and the light playing between them is thus rendered constant without the aid of complex regulators. In the experiments at the West India Docks the current was produced by a magneto-electric machine, worked by a small steam-engine, and the results are described as having been eminently satisfactory. For lights of small and medium size, an apparatus of even greater simplicity may be employed, the carbon points being dispensed with and nothing used beyond a piece of the so-called kaolin held between the electrodes. But M. Jablochkoff's prime improvement, which promises to greatly extend the use of the electric light, consists in his ability to divide the current, so as to supply several candles placed in the same circuit, each with its own coil. These candles may be of various degrees of illuminating power, and may be lighted or extinguished separately. In short, the electricity appears to be under such control, that it might be generated in some central establishment and laid on through wires to the several centres of illumination, just as freely as gas is at present distributed through pipes to any number of burners. MM. Denayrouze and Jablochkoff, who have employed the light in Paris, have described their process before the French Academy of Sciences.Comtes Rendus, No. 16, April 17, 1877.