Whitehead — Magnetic Effect of Electric Displacement. 109 



Akt. XY. — 2'he Magnetic Effect of Electric Displacement • 

 by John B. Whitehead, Je. 



HiSTOEicAL Review. 



In the development of liis theory of the electromagnetic 

 field, Maxwell assumes that the phenomenon of polarization in a 

 dielectric consists of an actual propagation or displacement of 

 charge in the direction of the polarization ; that in the case of 

 the charging of a condenser, for instance, this act of displace- 

 ment is equivalent to a current at any instant equal to the 

 rate of change of the surface charge on one of the plates, i. e. 



q=-7— -T-? <l being the current density, K the specific inductive 

 47r at 



capacity and F the difference in potential per unit length, and 



that therefore the current is continuous throughout the circuit. 



He assumes further that the displacement current has the same 



magnetic effect as would be produced by a conduction current 



of density $'=2— 77-? so that in the case of the condenser the 



magnetic effect in the neighborhood, incident upon any change 

 of charge, would be due to the combined influence of the cur- 

 rent in the charging wires and the displacement current in the 

 dielectric.^ " It appears, therefore, that at the same time that 

 a quantity Q of electricity is being transferred along the wire 

 by the electromotive force from B towards A, so as to cross 

 every section of the wire, the same quantity of electricity 

 crosses every section of the dielectric from A towards B by 

 reason of the electric displacement." (Art. 60.) 



" The variations of the electric displacement evidently con- 

 stitute electric currents." (Art. 60.) 



" One of the chief peculiarities of this treatise is the doctrine 

 which it asserts, that the true electric current C, that on which 

 the electromagnetic phenomena depend, is not the same thing 

 as K, the current of conduction, but that the time variation of 

 D, the electric displacement, must be taken into account in 

 estimating the total movement of electricity." (Art. 610.) 



The remarkable consequences of Maxwell's theory seem to 

 justify his assumptions beyond all question. There need only 

 be mentioned the finite velocity of propagation of electric and 

 magnetic actions, this velocity being the same as that of hght ; 

 the electromagnetic theory of light, which accounts for the 

 results of experiment, practically without exception ; the 



* Maxwell : Electricity and Magnetism, Art. 60, 75, 76, 111, 328-334, 608, 

 783, 791. 



Am. Jour. Sci.— Fourth Series, Yol. XIV, No. 80.— August, 1902. 



