Whitehead — Magnetic Effect of Electric Displacement. 1 27 



vent the effect sought. The first is a variation in the intensity 

 of the electric field from point to point and the tendency of 

 the dielectric to remain in the region of greatest intensity ; and 

 the second, the presence of the damj)ing vane. It is thought 

 that neither influence was great enough to mask a couple of 

 the value of that sought ; as has been stated, the rings were 

 carefully turned and adjusted in place so that their surfaces 

 were the same distance apart at all points. Indications of any 

 choice of position. by the beam in the electric field were of the 

 slightest, tlie stationary positions being very easily upset, and 

 often changing by one or two centimeters between readings. 

 The distribution of the electric field due to the coil, though 

 uncertain, could not have departed greatly from symmetry 

 about the center, and since the disturbance due to it manifested 

 itself in deflections rarely exceeding l'^™, it does not seem possi- 

 ble that the variations of intensity were great enough to account 

 for the absence of visible effect when the displacement couple 

 acting on the beam was reversed. In the earlier experiments 

 several series of observations were made without the damping 

 vane ; no difference in the behavior or in the nature of the 

 results was observed except the increase in time necessary for 

 the beam to become stationary. Observations could not be 

 made with the final form of the apparatus if the damper were 

 taken off ; the electric field was so nearly uniform that the 

 beam would not retain a stationary position definite enough to 

 be taken as a zero point. 



The following arrangement, independently conceived, but 

 afterwards discovered to be an improved modification of the 

 apparatus used by S. P. Thompson, offers a possible means of 

 detecting a magnetic eft'ect of the displacement current. An 

 Annulus l-Q^"" thick, built up in laminations of sheet u-on rings 

 S'lS*^"^ inside and 7"6*^™ outside diameter, was wound with a coil 

 of several hundred turns of fine wire, which was connected with 

 a telephone receiver. In the opening in this magnetic circuit 

 was placed a cylinder of dielectric 2'5tl:''"' in diameter and 2*54'^"^ 

 in length. Flat electrodes were brought very close to the ends 

 of the cylinder, but were not allowed to touch it. When now 

 an alternating E.M.F. is applied to the electrodes the resulting 

 displacement current should set up an alternating field in the 

 surrounding magnetic circuit, and so induce a current in the 

 telephone receiver. The space between the electrode and 

 dielectric prevents in great measure a possible conduction cur- 

 rent due to conductivity of the material of the dielectric. With 

 8800 volts at 133 cycles on the electrodes, neglecting the air 

 gaps, and with paraffin (K=2) as the dielectric, the displace- 

 ment current density is : 



