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The coherer has been displaced by a new device invented by Marconi, called a magnetic detector, by which the ether waves are aided by a stronger current to record the message. The effect is the same, but the method is entirely different. The sending of a long-distance message is a spectacular thing.
It consisted of two metal cylinders with leads attached, fitted tightly into the interior of a glass tube containing iron or steel filings. The instant an electric discharge of any sort occurred the coherer became conductive, and if it was tapped lightly its conducting property was immediately destroyed.
Though a small instrument, and more delicate than a clinical thermometer, it loomed large in the working-out of wireless telegraphy. To one plug of the coherer also was joined one pole of the local battery, while the other pole was in circuit with the other plug of the coherer through the recording instrument.
The tube of the coherer is generally of glass but any insulating substance will do; a wire enters at each end and is attached to little blocks of metal which are separated by a very small space. It is into this space the filings are loosely filled.
The tube of filings through which the electric current is made to pass in wireless telegraphy is called a coherer signifying that the filings cohere or cling together under the influence of the electric waves. Almost any metal will do for the filings but it is found that a combination of ninety per cent. nickel and ten per cent. silver answers the purpose best.
The coherer was still retained and by the end of 1900 enough had been accomplished to warrant Marconi in arranging for trans-Atlantic experiments between Poldhu, Cornwall and the United States, stations being located on Cape Cod and in Newfoundland. The trans-Atlantic transmission of signals was quite a different matter from working over 100 miles or so in Great Britain.
"I won't bore you, yet, with explanations of my radio-combinator, the telecommutator, the aerial coherer relay, and the rest of the technicalities of wireless control of dirigible, self-propelled vessels.
When shaken apart they again resisted the flow of current until it became powerful enough to cause them to again arrange themselves into a sort of bridge for its passage. Thus was the principle of the coherer discovered. An Italian scientist, Professor Calzecchi-Onesti, carried these experiments still further.
By this time a new device for the detection of messages was employed, as the coherer we have described even in its improved forms was found to possess its limitations of sensitiveness and did not respond satisfactorily to long distance signals.
It was here realized that various refinements in the receiving apparatus were necessary, and instead of the coherer a telephone was inserted in the secondary circuit of the oscillation transformer, and with this device on February 12th the first signals to be transmitted across the Atlantic were heard.
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