The Cardinal Discovery.
“Mr. Willoughby Smith, an engineer engaged in the laying of submarine cables, had devised a system of testing and signalling during their submersion. For this system, in 1872, it occurred to him that he might employ crystalline selenium, on account of its high resistance, at the shore end of a cable. On experiment the selenium was found to have all the resistance required; some of the bars displayed a resistance of 1400 megohms, as much as would be offered by a telegraph wire long enough to reach from the earth to the sun. But this resistance was found to be extremely variable; the reason was disclosed when Mr. May, an assistant, observed that the resistance of selenium is less in light than in darkness. This discovery created widespread interest throughout the world. Among the investigators who at once turned their attention to the subject was Professor W. G. Adams of King’s College, London, who proved that the action on selenium is chiefly due to the luminous rays of the spectrum, the ultra-red and ultra-violet rays having little or no effect. Dr. Werner Siemens, the eminent German physicist, produced a variety of selenium fifteen times more conductive in sunlight than in darkness. This extraordinary sensitiveness was brought about by heating for some hours at a temperature of 210° C., followed by extremely slow cooling.
Telephones receiving sounds through a beam of light.