120 



Dr. J. A. Ewing and Mr. L. H. Walter. [Jan. 11, 



" A New Method of Detecting Electrical Oscillations." By J. A. 

 Ewing, LL.D., E.R.S., and L. H. Walter, M.A. Eeceived 

 January 11, — Read February 11, 1904. 



The magnetic detector of Rutherford,* though now well known, 

 appears to have created little interest until attention was directed to 

 the subject by Marconi's adaptation of the method to his telephonic- 

 detector. Marconi's apparatus emplo}^ the change of hysteresis, 

 which is produced in iron by the influence of electric oscillations, 

 when these are caused to pass through a coil surrounding the iron, the 

 change being made manifest by means of a telephone. In his view, 

 the electric oscillations act by reducing the hysteresis.! 



It occurred to us to exhibit the alteration in hysteresis by a 

 different method, namely, by applying the principle which is used in 

 an instrument invented some years ago by one of us for the mechanical 

 measurement of hysteresis. In that instrument! the hysteresis is 

 measured by the mechanical couple between a magnetic field and the 

 iron, when either the iron or the magnet providing the field is caused 

 to revolve. Thus, if the field revolves, the iron tends to be dragged 

 after it, as a consequence of hysteresis in the reversals of its magnetism, 

 and if the motion is prevented by a spring or other control, it assumes 

 a deflected position. Suppose, now, the electric oscillations to act on 

 it, any change of the hysteresis caused by them will be exhibited by a 

 corresponding change in the deflection. We anticipated, in accord- 

 ance with the generally accepted view that hysteresis is reduced by 

 the oscillations, that their presence would be detected by a fall in the 

 deflection. 



With this expectation an experimental apparatus was arranged, 

 consisting of an electro-magnet, capable of being rotated on a vertical 

 axis by an electric motor. The magnet poles were bored out circular, 

 and between them was suspended, by a phosphor-bronze strip, a ring 

 made up of three thin, flat annuli of soft iron, clamped together, and 

 provided at the foot with an axial pivot. The ring was free to turn 

 inside of two bobbins wound with fine copper wire, the windings 

 being at right angles to the plane of the ring. Through these copper 

 windings, electrical oscillations, produced in the usual manner by 

 means of a distinct spark-gap, were passed. 



The first experiments resulted in a very small deflection from the 

 position due to normal hysteresis, indicating, as was expected, a 

 decrease of hysteresis when the oscillations arrived. The apparatus 



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



f ' Roy. Soc. Proc.,' vol. 70, pp. 341—344, 1902. 



X ' Journ. Inst. Electr. Engineers,' vol. 24, pp. 398—430, 1895. 



