﻿426 Mr. C. E. St. John on Wave-lengths 



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 

 character of iron wires exercises an important influence upon 

 the decay of electrical oscillations of high frequency, and that 

 currents of such frequency as occur in leyden-jar discharges 

 magnetize 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 negatives compared. 



Prof. J. J. Thomson has shown f that the presence of iron 

 can affect the rapidly oscillating electric discharges through 

 a rarefied gas by absorbing the energy of the discharges. 



In a paper upon the " Absorption Power of Metals for the 

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

 results that prove the great damping power of magnetic 

 metals upon electric oscillations of very high frequency 

 (100,000,000 double oscillations per second). 



If the damping power of iron is due to the fact that its 

 magnetic properties are brought into play under such rapidly 

 alternating forces, it still remains an interesting question 

 whether the self-induction of an iron circuit is measurably 

 greater than that of a similar copper circuit, and whether the 

 wave-length remains constant for oscillations of the same 

 period. 



In the determinations of the wave-length due to the 

 Hertzian vibrator, the arrangement originated by Hertz §, 

 and modified by Lecher |l and by Sarasin and De La Kive% 

 has been very generally employed. In this arrangement 

 secondary disks were placed face to face with the plates of 

 the vibrator and near to them, to each secondary disk a long 

 wire was attached, and these wires carried through the air 

 parallel to each other, with, sometimes, an additional disk on 

 the free ends. 



With such an arrangement no exact adjustment of the 

 length of the secondary circuit was required in order to excite 

 powerful oscillations in it, for the direct electrostatic induction 



* ' Proceedings of American Academy of Arts and Sciences/ xxv., 

 May 27, 1891. 



t Phil. Mag. [5] xxxii. p. 456, July 1891. 



% Wied. Ann. xliv. p. 74 (1891). 



§ Ibid, xxxiv. p. 551 (1888). 



j| Ibid. xli. p. 850. 



•f] Archives des Sciences Physiques, t. xxiii. p. 113 (1890). 



