. The science-history of the universe . coherer, Fig. 49, which consisted of a small glass tubecontaining two metallic plugs and separated by a gap par-tially filled with metallic filings. This is an example ofthe second class. The metallic filings, when looselypacked, offer a very high resistance to the passage of cur-rent through them, but the presence of waves breaksdown their contact resistance, which continues after thewaves have ceased. In order to again restore their re- WIRELESS TELEGRAPHY 315 sistance they must be tapped or shaken, an operationknown as decoherence. It is obvious that
. The science-history of the universe . coherer, Fig. 49, which consisted of a small glass tubecontaining two metallic plugs and separated by a gap par-tially filled with metallic filings. This is an example ofthe second class. The metallic filings, when looselypacked, offer a very high resistance to the passage of cur-rent through them, but the presence of waves breaksdown their contact resistance, which continues after thewaves have ceased. In order to again restore their re- WIRELESS TELEGRAPHY 315 sistance they must be tapped or shaken, an operationknown as decoherence. It is obvious that such an appa-ratus may be used like a key in a telegraphic circuit—a key operated by electric waves—and may therefore beused to operate a telegraphic instrument. Such was thefirst device used. It was defective, however, in that itwas necessary to tap it after each signal, decoherencewas not certain, it required frequent adjustment, and theresult was often a confused lot of arrangements of loose contacts were tried, and. Fig. 50 —The Lodge-Muirhead Coherer. the coherer was improved by Marconi, Lodge, Braun, andothers. One of the principal troubles being the opera-tion of decoherence, most of the inventors sought to de-velop a coherer which should be self-restoring, and anumber of successful types were invented. One of thesewas the Hughes coherer, employing carbon granulesplaced between iron plugs. The most perfect and suc-cessful of all these is, however, that devised by Sir OliverLodge and Dr. Muirhead, and used in the Lodge-Muirheadsystem. As shown in Fig. 50, it consists of a steel disk,slightly separated from a globule of mercury by a filmof oil, the disk being arranged to rotate slowly. The 316 ELECTRICITY presence of waves breaks down the oil film and estab-lishes contact with the mercury, which contact immedi-ately breaks upon the cessation of the waves. A siphonrecorder, placed in series with the cymoscope, is used torecord the message. Altho a number
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