506 Displacement Currents [OH. xvn 



solution reduces to that of Liouville on taking the surface S to be a sphere, 



a a 



As with the former solutions, the result obtained clearly indicates propa 

 gation in all directions with uniform velocity a. 



PROPAGATION OF ELECTROMAGNETIC WAVES. 

 581. It is now clear that the system of equations 



d 2 X ~ 



etc. obtained in 577 indicate that, in a homogeneous isotropic dielectric, all 

 electromagnetic effects ought to be propagated with the uniform velocity 



C 



This enables us to apply a severe test to the truth of the theory of 



displacement-currents. The value of C can of course be determined experi- 

 mentally, and the velocity of propagation of electromagnetic waves can also 

 be determined. In air, in which K = fj, = l, these two quantities ought, if 

 the hypothesis of displacement-currents is sound, to be identical. 



For the value of C, the ratio of the two units, the following experimental 

 results are collected by Abraham *, as likely to be most accurate : 



Himstead 3-0057 x 10 10 



Rosa 3-0000 x 10 10 



Abraham 2'9913xl0 10 



Pellat 3-0092 x 10 10 



J.J.Thomson 2*9960 X 10 10 Hurmuzescu 3'OOlOxlO 10 



Perot and Fabry 2'9973 x 10 10 . 



The mean of these quantities is 



C = 3-0001 xlO 10 . 



For the velocity of propagation of electromagnetic waves in air, the 

 following experimental values are collected by Blondlot and Guttonf. 



Blondlot 3-022 xlO 10 , 2'964xl0 10 , 2*980 x 10 10 



Trowbridge and Duane 3-003 x 10 10 



MacLean 2-991 IxlO 10 



Saunders 2-982 xlO 10 , 2-997 x 10 10 . 



The mean of these quantities is 2'991 x 10 10 . 



Thus the two quantities agree to within a difference which is easily within 

 the limits of experimental error. 



Both these quantities are equal, or very nearly equal, to the velocity 

 of light, and this led Maxwell to suggest that the phenomena of light 

 propagation were, in effect, identical with the propagation of electric waves. 



* Eapports presentes au Congres de Physique, Paris, 1900. Vol. n, p. 267. 

 t I.e. p. 283. 



