. Discovery. Science. DISCOVERY 123 The principle upon which this instrument is based depnds upon the vagaries of the substance selenium, an element which is closely allied chemically to sulphur, with which it is often associated in nature, and which is regarded as an unmitigated nuisance by the manu- facturer of sulphuric acid, because it spoils his acid for use in the paper-making industry. Selenium was formerly obtained as a waste product from the lead- chamber process of making this acid, but it is now obtained almost exclusively from the electrolytic smelting of copper, which is carried o
. Discovery. Science. DISCOVERY 123 The principle upon which this instrument is based depnds upon the vagaries of the substance selenium, an element which is closely allied chemically to sulphur, with which it is often associated in nature, and which is regarded as an unmitigated nuisance by the manu- facturer of sulphuric acid, because it spoils his acid for use in the paper-making industry. Selenium was formerly obtained as a waste product from the lead- chamber process of making this acid, but it is now obtained almost exclusively from the electrolytic smelting of copper, which is carried on chiefly in the United States. The fact that is made use of is that the electrial conducti\-ity of selenium in one of its physical forms varies greatly in accordance with the amount of light which falls upon it. This was first noticed by Willoughby Smith and his assistant May, who were carrying out some cable-testing experiments on the Island Station Valentia, off the coast of Ireland, in the year 1873. May could not get the results from his instruments by day to agree with those by night. He therefore came to the conclusion that there must be something peculiar in the way this element selenium behaved when an electric current was passed through it. It had long been noticed that selenium offered a very great resistance to the passage of the current, but it had not been observed that the degree of resistance was affected by the amount Of light falling on it. It is this peculiarity of selenium that is the basis of the optophone. Writing to a friend early in 1873, Willoughby Smith tells of his discovery in rather a witty and graphic manner: IQ09 first produced one of the up-to-date cells, and Fournier d'Albe followed with his two years later. Presser's cell consisted of a circular piece of steatite (a silicate of magnesium) covered with platinum and engrav^ed in concentric rings. The whole surface was then covered with molten selenium. Fournier d'Albe's cell is simpler still, and
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