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



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

 netic 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 lengths due to the Hertz- 

 ian vibrator, the arrangement originated by Hertz* and modi- 

 fied by Lecherf and by Sarasin and De La Rive;}: 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 power- 

 ful oscillations in it, for the direct electrostatic induction 

 between the plates of the secondary and the disks on the ends 

 of the primary wires was so great that vigorous oscillations 

 were produced along the secondary wires whatever their length 

 might be, and several systems of waves could be detected which 

 seemed to give experimental grounds for believing that the 

 wave system sent out from the Hertzian vibrator was very 

 complex. 



The capacity of the vibrator is increased by the presence of 

 these secondary disks so near to the vibrator plates, so that the 

 wave length found under these conditions is not that due to 

 the simple Hertzian vibrator but that due to a very complex 

 oscillating system with somewhat obscure internal reactions. 

 Especially is this true when the wires are bridged as in the 

 Lecher arrangement. The latter calls attention to the change in 

 the sound of the primary spark when the secondary wires are 

 bridged by a conductor. There is a very marked difference in 

 the spark when the secondary circuit is removed entirely. 

 The- spark then loses much in body and explosive character. 

 The secondary circuit under these conditions exerts apparently 

 a strong reaction upon the primary. 



It seemed desirable to devise some form of secondary depend- 

 ing more directly ivpon the principle of electrical resonance, 

 the use of which would not increase the capacity of the vibrator 

 and whose reaction upon it would be a minimum. This was 

 done by omitting the secondary disks and using simply a single 

 long wire as shown in fig. la. 



* AnnaleD, xxxiv, p. 551, 1888. 



f Annalen, lxi, p. 850. 



\ Archives des Sciences physiques, t. xxiii, p. 113, 1890. 



