( fi23 ) 



field, will he acted on by a similar force as a bod}' carrying a con- 

 duction-current. In Wmitkhkad's apparatus two cylindric metallic 

 plates, having the same vertical axis P (2, formed a condenser, 

 in which a rapidlv alternating electric field was maintained; at 

 the same time alternating currents were passed through the horizontal 

 windings of a circular coil, surrounding the condenser; the axis of 

 the coil, which is at the same time the axis of its magnetic field, 

 coincided with /■* Q. A sensitive torsion-l)aiance Avas suspended by a 

 wire passing along the axis of the histrument; the ends of the beam 

 carried each a piece of some solid dielectric, so that these two equal 

 pieces hung, diametricall}' opposite each other, in the air-space 

 between the condenser-plates. The two fields, the electric and the 

 magnetic, had exactly the same period, being produced by the same 

 alternate current-machine ; besides, the arrangements were such that 

 there was a phase-difference of a quarter period between the two 

 fields. Thns, at the instants at which the magnetic force had its 

 maximum \'alues, the rate of change of the electric field and conse- 

 ({uently the intensity of the displacement-current ^vas like\vise at its 

 maximum. Under these circumstances a sensible cou})le acting on the 

 dielectric was expected, but no deviation of the beam, attributable 

 to such a coiq^le, could with certainty be observed. 



We may remark in the first place that in Whitehead's formula 

 for the expected effect, the specific inductive capacity K ai)pears in 

 the numerator. If this were right, a couple would act on the aether 

 between the plates itself. According to the theory of electrons, as here 

 presented, ponderomotive force acts onlj' on the electrons contained 

 in ponderable bodies, but in no case on the aether. The theory 

 therefore regards every ponderomotive action as due to the difference 

 between the properties of the boily acted upon and the aether; it 

 can lead to a formula containing in the numerator K — J, but never 

 to one, containing, instead of this factor, the coefficient K itself. 



In the second place Whitehead has oxerlooked a circumstance by 

 which the effect he sought for nnist have been, at least for the greater 

 part, compensated. The compensation may be shown to be complete 

 if the properties of the dielectric used differ tVoui those of the aether 

 to so small extent, that quantities which are bi this respect of the 

 second order of magnitude, i. e. of the order {K — J)^ may be neglected. 



If this may be done, the ponderomotive action on a ponderable 

 dielectric, placed I)etween the condenser-plates, may bo considered not 

 to be altered by the presence in the field of a second or third piece 

 of the same dielectric. Now, the two bodies suspended at the ends of 

 Whitehead's torsion-balance may be taken to have been parts of a 



42 



Proceedings Royal Acad. Amsterdam. Vol. V. 



