81 



On the Use of Xickkl in the Coke of the Marconi 

 Magnetic Coherer. 



By Arthur L. Foley. 



Tlie luaguetic detector of electric waves, descrilted and used by 

 IMarcoiii,* consisted of a "core or rod of thin iron wires on which were 

 wound one or two hayers of thin insulated copper wire. Uver this 

 winding insulating material was placed, and over this again, another 

 longer winding of thin copper wire contained in a narrow bobbin." 

 One terminal of the inside M'inding was connected to earth, the other 

 to an elevated conductor. The ends of the outside winding were con- 

 nected to a telephone. A horseshoe magnet, suitablj' placed, was moved 

 by clockwork so as to cause a continuous change or successive reversals 

 or the magnetism of the iron core. Electric oscillations of suitable 

 period appeared to reduce the effects of magnetic hysteresis, hence the 

 magnetism of the iron core increased or decreased suddenly with each 

 spark of the transmitter, inducing a current in the outer winding con- 

 nected to the telephone. Marconi had (..Tune, 1UU2) used this apparatus 

 for some months in the reception of wireless telegraph messages over 

 a distance of l.i^ nnles, and with less power employed at the transmit- 

 ting station than would have been required had he used a reliable coherer 

 instead of the magnetic detector. 



Marconi noticed that "the signals in the telephone are weakest when 

 the poles of the rotating magnet have just passed the core and are 

 increasing their distance from it, whilst they are strongest when the 

 magnet poles are approaching the core." To obtain more definite results 

 on this point I arranged to use a ballistic galvanometer instead of a 

 telephone, and to take readings for various determined positions of 

 the magnet and core. 



The core, which was 5 cm. long, consisted of twenty-six pieces of 

 annealed piano wire, .003 cm. in diameter. Over this was wound a 

 single layer of two hundred turns of silk insulated copper wire No. 3G, 

 giving a total diameter of core and coil of approximately .4 cm. One 

 end of the coil was connected to a vertical wire 200 cm. long; the other 

 end was put to earth. 



"Note "n a .Magnitie Detector of Electric Waves. Uj- G. Marconi, Proceedings of tho 

 Royal Society, Vol. LXX, Nu. MV.',, July 29, 1902, 

 C— A. OF SciKXCK. '03. 



