St. John — Wave lengths of Electricity on Iron Wires. 31 



Art. XLV. — Wave lengths of Electricity on Iron Wires ; 

 by Charles E. St. John, A.M. With Plate IX. 



The question whether the magnetic properties of iron are 

 called into play under extremely rapid alternations of the mag- 

 netizing forces has been an interesting one, and has received 

 various answers. 



Hertz found negative results when he replaced one side of a 

 rectangular copper resonater* by an equal iron wire, and in a 

 later paper on the " Finite velocity of Electro magnetic 

 Action,"f when he compares the rate of propagation along cop- 

 per and iron wires, he concludes " that the rate of propagation 

 in all wires is the same, and we are justified in speaking of it 

 as a definite velocity. Even iron wires are no exception to 

 this general rule ; hence the magnetic properties of the iron 

 are not called into play by such rapid disturbances."^: 



Dr. Oliver J. Lodge attacked the question by means of his 

 experiment on the alternate path. In his " Lightning Conduc- 

 tors and Lightning Guards " (1892)§ he remarks : " But everyone 

 will say — and I should have said before trying — surely iron 

 has more self-induction than copper. A current going through 

 iron has to magnetize it in concentric cylinders, and this takes 

 time. But experiment declares against this view for the case 

 of Leyden jar discharges." 



Prof. John Trowbridge has shown that the magnetic charac- 

 ter of iron wires exercises an important influence upon the 

 decay of electrical oscillations of high frequency, and that cur- 

 rents of such frequency as occur in Leyden jar discharges mag- 

 netize the iron. The spark in geometrically similar oscillating 

 circuits of copper and iron was photographed by means of a 

 revolving mirror and the number of oscillations on the nega- 

 tives compared. 



Prof. J. J. Thompson has shown | that the presence of iron 

 can affect the rapidly oscillating electric discharges through a 

 rarified jar by absorbing the energy of the discharges. 



In a paper upon the Absorption Power of Metals for the 

 Energy of " Electric Waves," V. Bjerknes has also given 

 results that prove the great damping power of magnetic metals 

 upon electric oscillation of very high frequency (100,000,000 

 double oscillations per second). 



* Annalen xxxi, p. 429, 1887. f Annalen. xxxiv, p. 351, 1888. 



% Electric Waves, p. 113. 



§ Proceedings of American Academy of Arts and Sciences, vol. xxv, May 27, 

 1891. 



|| Phil. Mag. (V), lxxxii, p. 456, July, 1891. 



