Magnetic Detector of Electric Waves 



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" Note on a Magnetic Detector of Electric Waves, which can 

 be employed as a Eeceiver for Space Telegraphy." By 

 G. Marconi, M.I.E.E. Communicated by Dr. J. A. Fleming, 

 F.R.S. Received June 10 —Read June 12, 1902. 



The present note bears upon the special manner in which a core or 

 rod of iron or steel placed in a varying magnetic field is affected by 

 high-frequency oscillations transmitted from considerable distances. 



The magnetisation and demagnetisation of steel needles by the effect 

 of electrical oscillations has long been known, and has been noted 

 especially by Professor J. Henry, Abria, Lord Rayleigh, and others. 

 Mr; E. Rutherford also has described a magnetic detector of electric 

 waves, based on the partial demagnetisation of a small core composed 

 of fine steel needles, previously magnetised to saturation, and placed 

 in a solenoid of fine copper wire connected to exposed plates. By 

 means of a magnetometer Mr. Rutherford succeeded in tracing the 

 effects of his electrical radiator up to a distance of f mile across Cam- 

 bridge.* 



The detector which I am about to describe is, in my opinion, based 

 upon the decrease of magnetic hysteresis which takes place in iron 

 when, under certain conditions, it is exposed to the effect of high- 

 frequency or Hertzian waves. 



As employed by me up to the present, it has been constructed in the 

 following manner : — On a core or rod consisting of thin iron wires are 

 wound one or two layers of thin insulated copper wire. Over this 

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

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



The ends of the winding nearest the iron core are connected to the 

 plates or wires of the resonator, or as is the usual practice in long- 

 distance spaee telegraphy, to earth and to an elevated conductor • or 

 they may be connected to the secondary of a suitable receiving trans- 

 former or intensifying coil, such as are now employed for syntonic 

 wireless telegraphy. The ends of the other winding are connected to 

 the terminals of a telephone or other suitable receiving instrument. 

 Near the ends of the core, or in close proximity to it, is placed a 

 magnet, preferably a horse-shoe magnet, whichj by a clockwork 

 arrangement, is so moved or revolved as to cause a slow and constant 

 change, or successive reversals, in the magnetisation of the iron core. 

 I have noticed that if electrical oscillations of suitable period be sent 

 from a transmitter according to the now well-known methods, rapid 

 changes are effected in the magnetisation of the iron wires, and these 



* See ' Phil. Trans.,' A, vol. 189 (1897), pp. 1—24. 



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