Masterpieces of Science 



The coherer, as improved by Marconi, is a glass 

 tube about one and one-half inches long and 

 about one-twelfth of an inch in internal diameter. 

 The electrodes are inserted in this tube so as 

 almost to touch; between them is about one- 

 thirtieth of an inch filled with a pinch of the 

 responsive mixture which forms the pivot of 

 the whole contrivance. This mixture is 90 per 

 cent, nickel filings, 10 per cent, hard silver filings, 

 and a mere trace of mercury; the tube is ex- 



Fig. 71. Marconi coherer, enlarged view 



hausted of air to within one ten-thousandth part 

 (Fig. 7 1) . How does this trifle of metallic dust 

 manage loudly to utter its signals through a 

 telegraphic sounder, or forcibly indent them 

 upon a moving strip of paper? Not directly, 

 but indirectly, as the very last refinement of ini- 

 tiation. Let us imagine an ordinary telegraphic 

 battery strong enough loudly to tick out a mes- 

 sage. Be it ever so strong it remains silent 

 until its circuit is completed, and for that com- 

 pletion the merest touch suffices. Now the 

 thread of dust in the coherer forms part of such 

 a telegraphic circuit: as loose dust it is an effect- 

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