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PHOTO-TELEGRAPHY 



plugs near together, a part of the intermediate 

 space being filled with silver and nickel filings. 

 When a current is received these filings cohere and 

 make tolerable contact, so that the wires connected 

 to the silver plugs will convey a current. The co- 

 hesion requires then to be destroyed by a tap or 

 knock, when the coherer again becomes a sensitive 



FIG. 59. Diagram of Marconi's Electromagnetic 

 Detector. 



detector. A far more satisfactory arrangement 

 devised by Marconi for detecting the signals is seen 

 in Fig. 59. Here a string made of fine soft-iron 

 wires travels round two discs D, D, which are 

 actuated by clockwork. A glass tube fits over a 

 small portion of the travelling wire, round which is 

 wound a fine wire coil, the ends of this coil being 

 connected to the antenna and earth plate respec- 

 tively. A small secondary coil S wound over the 



