Abstract
ON being called upon by the chairman to show his experiments, Prof. Ayrton stated that he and Mr. Perry thought that the occasion of the reading of Mr. Bidwell's paper was a suitable one for their showing to the Society that they were constructing the apparatus described by them in a letter in NATURE, vol. xxii. p. 31. The feasibility of their plan had been combated, and at the last meeting of the British Association at Swansea it was confidently asserted that the action of selenium was not quick enough to register rapid changes of light intensity—an idea, however, which they stated in the discussion at the time there was experimental evidence to disprove. After that came the publication and exhibition of the photophone, proving that seleaiuin changed its electrical properties synchronously with rapid changes in light intensity. For a light telegraph however not only was this property necessary, but in addition that the electric changes in the selenium should be considerable for a comparatively small change in the light. They had, therefore, tried to make sensitive selenium cells of low resistance. The method they had employed consisted in winding two wires parallel on strips of box-wood, ivory, and other non-conductors in section, somewhat like that of a paper-knife in the manner subsequently described by Mr. Bidwell in NATURE, but they had not found it necessary to cut a screw on the wood or mica in a lathe. Of the twenty-five cells that they had constructed they had invariably found, like Mr. Bidwell, that only those were sensitive that had a high resistance. They were aware that Prof. Adams had made sensitive cells of low resistance, and had he been present they would have liked to ask whether it was not only for very small electromotive forces that the cells were sensitive. They had also found that when sensitive cells of 100,000 ohms resistance diminished in resistance to only a few hundred ohms by natural annealing extending over some months, the cells lost entirely their sensibility. Further that certain sensitive cells of high resistance were sensitive as long as an electromotive force of not more than about seven volts was employed to send a current through them, but for electromotive forces much above this the cells were comparatively unsensitive to light, but the sensibility was not destroyed for electromotive forces smaller than seven volts used subsequently. These phenomena, which they believed had not been previously noticed, pointed, they suggested, to the sensibility of selenium being due almost entirely to a polarisation and not merely to a change of resistance, as was commonly supposed and stated. Might it not be possible, they asked, that there was an electromotive force developed iu selenium by light, which, for different cells, increased more rapidly than the resistance of the cell, and which was the greater, the greater the electromotive force of the auxiliary battery employed; that in fact selenium became rapidly polarised by the auxiliary current flowing through it, and that this polarisation, the amount of which depended on this current, was removed in proportion to the intensity of the light. That a small electromotive force was developed in selenium by light when no auxiliary current was sent through it, had been conclusively shown by Prof. Adams and Mr. Day in 1876, a result that they had also experienced; and they would mention that a careful examination which they had recently made of the paper published by Prof. Adams and Mr. Day in the Phil. Transactions for 1876, showed that if we assumed all the instances therein mentioned of sensibility of selenium to light were due to an electromotive force set up, and not to change of resistance at all, then on the whole all the results would have been arrived at if this electromotive force set up in different cells, for the same intensity of light, increased more rapidly than the resistance of the cell, and was the greater, the greater the electromotive force of the auxiliary battery employed. They disagreed therefore from Mr. Bidwell in his idea that the name “cell” was at all inappropriate.
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Seeing by Electricity 1 . Nature 23, 423–424 (1881). https://doi.org/10.1038/023423a0
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DOI: https://doi.org/10.1038/023423a0