164 Dr. W. F. Gr. Swann on the Magnetic Field produced 



graphically, the dips in the middle being due to the brass 

 axle which passed through the centre of the rotating system. 

 From these curves we find for a difference of 4000 volts 

 between the central and external condenser plates, the values 

 Vx = 588 volts V 2 =543 volts. A slight error in the deter- 

 mination of the positions of the maxima of the curves is not 

 of great importance in view of the sharpness of the maxima. 

 The slight difference in the values of V for the two coils is. 

 not surprising in view of the large shielding effect of the 

 earthed coils, and consequent rapid variation of the potential 

 in the vicinity of, and in a direction perpendicular to, the 

 surfaces of the dielectrics along which the potentials were 

 measured. 



Table V. Table VI. 



E. 



a. 



V 



Volts. 



y- 



4000 



22 



530 



0-8 



... 



37 



665 



1-5 





56 



790 



20 





28 



590 



3-0 





28 



590 



4-0 



... 



28 



590 



5-0 





47 



733 



6-0 





56 



790 



6-5 





30-5 



610 



7-2 



-. 



a. 



V 



Volts. 



U- 



; 4000 



27 



580 



0-8 





53 



770 



1-5 





36 



655 



20 





22 



530 



30 





13 



405 



4-0 





26 



575 



50 





45 



720 



60 





47 



733 



6-5 





23 



540 



7-2 



Section 4. The Nature of Dielectric Action. 



It may at once be remarked that the theorem proved in 

 section 1 shows that if the whole of dielectric action is to be 

 attributed to electric charges in the dielectric, the motion of 

 the system through space cannot produce a magnetic flux 

 through a closed circuit kept at constant potential. It is, I 

 think, nevertheless interesting to verify the conclusion by a 

 rather closer examination of the phenomenon of dielectric 

 action, with a view to a clearer physical coordination of the 

 various quantities involved. 



We will first define specific inductive capacity in the 

 following way. Let there be two large parallel condenser 

 plates C, D either moving through space or at rest ; let one 

 plate be charged and the other earthed. Let V x be the 

 potential difference between two planes A and B situated 

 between and parallel to the two plates. Now fill the space 

 between A and B with a slab of material, and let Y 2 be the 

 new potential difference between A and B. Then the specific 



V, 



inductive capacity of the material is K= ^-. 



