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Prof. J. A. Fleming. Magnetic Detector for [Feb. 11, 



" A Note on a Form of Magnetic Detector for Hertzian Waves, 

 adapted for Quantitative "Work." By Dr. J. A. Fleming, 

 F.E.S., Professor of Electrical Engineering in University 

 College, London. Pieceived February 11, — Read March 5, 

 1903. 



The known power of electrical oscillations to demagnetise iron or 

 steel was first applied in the construction of a detector of Hertzian 

 waves, as far as the author is aware, by Mr. E. Rutherford.* The 

 power possessed by electrical oscillations to annul the magnetic 

 hysteresis of iron was discovered by Mr. G. Marconi and applied by 

 him in the construction of his ingenious and extraordinarily sensitive 

 Hertzian wave detector, for use in connection with wireless tele- 

 graphy.! 



The following note describes a form of magnetic Hertzian wave 

 detector, which has been constructed by the writer for the purpose of 

 quantitative experiments in connection with Hertzian waves. 



Every one who has experimented with a Hertzian oscillator, or 

 electric wave radiator in any form, involving a spark gap, is well 

 aware of the immense differ ence in the radiative power produced by 

 slight alterations in the nature of the spark or the spark balls, and has 

 felt the want of some instrument which shall indicate and measure 

 exactly the intensity of the radiation. As a receiving instrument, the 

 coherer or sensitive imperfect contact is of very little use quantita- 

 tively, because its indications are influenced by very slight accidental 

 changes at the contact or contacts. Thus, the sensitiveness of the 

 metallic filings coherer depends upon the manner in which it was left 

 after its last use, and by the mode in which it is tapped or shaken, 

 and the change in the conductivity which it experiences on the impact 

 of an electric wave, is variable and uncertain. Hence, although sensi- 

 tive as a mere wave detector, the coherer is of little or no use in 

 quantitative work. On the other hand, the magnetic detector is not 

 only superior to the coherer in sensitiveness when properly con- 

 structed, but is capable of being used as a measuring instrument. Id 

 the form in which it was constructed by Mr. Rutherford, an extremely 

 fine bundle of iron or steel wires was magnetised by means of a 

 magnet, or by being placed in the interior of a solenoid, and then 

 demagnetised by an electrical oscillation passing through another coil 



* See Mr. E. Rutherford, " On a Magnetic Detector of Electric Waves and 

 some of its applications," ' Roy. Soc. Proc.,' 1896, rol. 60, p. 1S4; see also 'Phil. 

 Trans.,' A, 1897, vol. 189, p. 1. 



f Mr. G-. Marconi, " IS~ote on a Magnetic Detector for Electric Waves which 

 can be employed as a Keceiver for Space Telegraphy," ' Roy. Soc. Proc.,' 1902, 

 toI. 70, p. 341. 



