110 Whitehead — Magnetic Effect of Electric Displacement. 



experiments of Hertz on electric waves showing them to be 

 identical in behavior with light waves. The direct magnetic 

 effect of the displacement current, however, has never been 

 satisfactorily observed, if at all ; pubhshed accounts of work in 

 this direction seem to be limited to those of Rontgen, S. P. 

 Thompson, Nicolaieff and Blondlot. 



Rontgen. — The earliest attempt, apparently, was that of 

 Rontgen in 1885.* He rotated a rubber disc between two 

 stationary glass j)lates all in horizontal planes. The upper 

 plate was coated with tin-foil which was grounded ; the lower 

 plate had on it a ring of tin-foil which was split along a diam- 

 eter and the two halves oppositely charged. As the disc 

 rotated there was a change in polarization at those portions of 

 it passing over the opening in the tin-foil, and the resulting 

 displacement current was in opposite directions at the two 

 ends of this opening. Over the whole system and as close as 

 possible to the upper jDlate, he suspended an astatic needle 

 whose direction was along the line of the opening in the tin 

 foil, and the line of suspension the continuation of that of the 

 axis of the rotating disc. The length of the lower needle 

 brought its ends to the center of the width of the tin-foil rings ; 

 any magnetic effect of the displacement current would thus 

 tend to deflect the astatic system. On commutating the 

 charges on the rings the deflection read by a mirror and scale 

 was never over V^^^^ and the needle was subject to an oscilla- 

 tion of that amount due to disturbing influences. The ob- 

 server, being ignorant of the direction of commutation, was 

 supposed to take into account the motion already possessed by 

 the needle in giving the direction of the resulting impulse. 

 Rontgen states that after 1000 observations he acquired such 

 practice as to be able to observe the proper direction nearly 

 every time. 



The results of this work can hardly be considered conclu- 

 sive. That little importance is to be credited to it is evidenced 

 by the small notice it has attracted. The genuineness of the 

 observed deflection is particularly to be questioned in view of 

 the fact that the poles of the needle were not in proper posi- 

 tion to experience the full magnetic force of the displacement 

 current. Each pole was entirely above the plane of the sur- 

 face of the dielectric, and it may well be questioned what is 

 the distribution of the magnetic field beyond the terminating 

 surface of an '' open current." 



Thompson.— hi 1889 S. P. Thompsonf wound an iron ring 

 wdth many turns of fine wire and imbedded it in a block of 

 paraffin having on two sides parallel to the plane of the ring 



*Rep. der Physik, No. 21, 1885, p. 521. 

 t Proc. Roy. Soc, xlv, p. 392, 1889. 



