THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XXI. — The Velocity of the Propagation of Magnetism • 

 by Henry A. Perkins. 



The effect of self-induction in a circuit causing a retardation 

 in the establishment of a magnetic flux is of course well known 

 to everyone ; but an allied effect, caused not by the inductance 

 between coil and magnetic circuit, but in the magnetic circuit 

 itself, thus causing a very perceptible interval of time between 

 the appearance of the magnetism at two points, has not been so 

 thoroughly studied, although it must be present in all alternat- 

 ing current machinery to a certain degree. This retardation is 

 thus due to the eddy currents in the core of the magnet in ques- 

 tion and is a complicated function of the various physical prop- 

 erties of the core, the frequency and magnitude of the impressed 

 electromotive force. 



This phenomenon was first investigated in detail by Ober- 

 bech* in 1884. His apparatus consisted in a core with mag- 

 netizing coil. Beside this coil were two others whose distance 

 apart could be varied at will. To measure the phase angle 

 between the electromotive force generated in these coils when 

 a sinusoidal e.m.f . was impressed on the main coil, be made use 

 of a dynamometer, the fixed coil being in series with one of the 

 sliding coils, and the movable one with the other. The result- 

 ing deflection he showed to be a function of various known 

 quantities and some unknowns that were eliminated by repeat- 

 ing the experiment with added resistance in series with one of 

 the two circuits, thus obtaining a different deflection. This was 

 repeated at different distances up to 20 cm . His results show 

 values of the lagging angle from 2° 16', when the distance was 



* Wied. Ann., xxii, p. 73. 



Am. Jour. Sci. — Fourth Series, Vol. XVIII, No. 105. — September, 1904. 

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