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Updated: May 4, 2025
The optophone translates light into sound by means of that wonderful little element, selenium, which in darkness is a poor conductor of electricity, but in light is a good conductor. This property is used in the optophone in transmitting an electric current which is interrupted by a special clockwork interrupter. It makes light and darkness audible in the telephone.
Thus bismuth and antimony give a much stronger current with the same heating than copper and iron. Bismuth and selenium produce the best result, but selenium is expensive and not easy to manipulate. Copper and German silver will make a cheap experimental couple:
The result has been, on the one hand, confusion of conductors beyond a certain distance, with the absolute impossibility of obtaining perfect insulation; and, on the other hand, an utter want of synchronism. The unequal and slow sensitiveness of the selenium likewise obstructed the proper working of the apparatus.
Tainter, on the top of the schoolhouse, almost blinded by the light which was coming in at the window of my laboratory, and vainly trying to understand the gestures he was making to me at that great distance, the thought occurred to me to listen to the telephones connected with the selenium receiver. Mr. Tainter saw me disappear from the window, and at once spoke to the transmitter.
"Selenium," went on Tom, "was discovered in 1817, by J. J. Berzelius, and he gave it that name from the Greek word for moon, on account of selenium being so similar, in some ways, to tellurium. That last is named after the Latin word tellus, the earth." "Do they dig it?" Ned wanted to know.
Neither did it require aerial masts or wires and a trained telegrapher who could send and receive the telegraph code. It was put to some use between battle-ships and smaller craft lying within a radius of a few miles. The sensitive selenium cell proved unreliable, however, and this means of communication was destined to failure.
"My theory is that the moon rock is of some composition sensitive to the action of moon rays; somewhat as the metal selenium is to sun rays. The little circles over the top are, without doubt, its operating agency. When the light strikes them they release the mechanism that opens the slab, just as you can open doors with sun or electric light by an ingenious arrangement of selenium-cells.
The current from the battery, B, traverses the selenium plates and the bobbins of the magnet, M, and returns to B through the rheostat, W; and the same occurs with the current from B'. The two currents, then, are absolutely independent of one another. From this description it is very easy to see how the system works.
"Koku was near the plate in the other booth when Rad put on the double current. There was a light there, and in an instant his likeness was sent over the wire, and imprinted on this plate. Now let's see what we can do. You go to that other booth, Ned. I'll see if I can get your picture, and send you mine. Here, take some extra selenium plates along. You know how to connect them."
The apparatus consisted of a transmitter with a mouthpiece, conveying the sound of the voice to a silvered diaphragm or mirror, which reflected the vibratory beam through a lens towards the selenium receiver, which was simply a parabolic reflector, in the focus of which was placed the selenium cells connected in circuit with a battery and a pair of telephones, one for each ear.
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